# ARI Institute Colloquium

### Upcoming events

2023-02-09
11:15
Stellar feedback shaping the ionized ISM in the nearby galaxies
Oleg Egorov (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Stellar feedback plays an important role in the regulation of the structure, kinematics and chemical abundance of the interstellar medium. Multiple stellar winds and supernovae explosions inject energy and momentum into the interstellar medium. These processes are probable drivers of the turbulence of the ISM. They can also create large holes and superbubbles with sizes varying from several pc to several kpc. Modern integral-field spectrographs allow us to study the resolved properties of the ionized gas, and the high-resolution images from space base telescopes provide information about young stars in the nearby galaxies. Thus, using the combined data set, we can connect directly the ionized gas to the young stellar population. In my talk, I will focus on how the mechanical feedback impacts the small-scale morphology and kinematics of the ISM of nearby galaxies as observed in the PHANGS-MUSE and PHANGS-HST data. Also, I will consider how the feedback from young stars affects the dust content in the nearby galaxies based on the new PHANGS-JWST observations.

2023-02-16
11:15
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Walter Dehnen (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
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### Past events

2023-02-02
11:15
Population Analysis of Short Gamma-ray Bursts and Magnetar Giant Flares
Elisa Schoesser (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Magnetars are highly-magnetized (ca. 1016 G) neutron stars which may randomly emit bursts of high energy radiation, in the form of hard X-rays or much more rarely giant flares of gamma-rays during their active phase. Recent observations of GRB 200415A, a short and very bright pulse of gamma-rays, have been claimed to be an extragalactic magnetar giant flare (MGF) whose host galaxy is the Sculptur Galaxy (NGC 253). However, as the redshift of the transient object was not able to be measured, it is possible that the measured location of the transient on the celestial sphere and the location of the local galaxy merely coincide. Thus, its real progenitor could have been arbitrarily far away, leaving the standard model of short gamma-ray bursts (SGRBs), the merger of two compact objects, as an explanation for the observations. In this talk, I will present an estimate of the false-alarm rate of SGRBs being incorrectly identified as MGFs using population synthesis to simulate data collected by the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope.

2023-01-26
11:15
Stellar associations powering HII regions
Fabian Scheuermann (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Newly formed stars will ionize and enrich the surrounding gas and thereby contribute to regulating the star formation cycle. However, fragmentation of the cloud means that stellar feedback is not perfectly coupled to the gas. The PHANGS survey provides a large sample of galaxies that is perfectly suited to study the different phases of the ISM, which is necessary to put constraints on this process. With PHANGS-MUSE we detect over 24,000 HII regions across 19 galaxies and with PHANGS-HST we identify a catalogue of over 15,000 compact stellar associations. Both catalogues are matched and we obtain a sample of 4,177 clearly matched HII regions and ionizing clusters. By comparing the observed Halpha flux to the number of ionizing photons predicted by stellar population synthesis models, we can quantify the leaking radiation. This allows us to measure the escape fraction for one of the largest samples of HII regions in the literature and study correlations with other properties of the cloud.

2023-01-19
11:15
What is the origin of dust in elliptical galaxies? - M87 and NGC 4696
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Although dust in elliptical galaxies is ubiquitous, its origin is not well understood, given the absence of young stellar populations with classical dust producers like post-AGB stars or supernovae. Many authors still promote an “external” origin through galaxy mergers. However, at least in one case, we have shown that the dust has its origin in nuclear dusty outflows (NGC 1316 or Fornax A). This talk presents two more key galaxies: M87, the central galaxy of a Virgo subcluster, and NGC 4696, the central galaxy of the Centaurus galaxy cluster. The discussion is based on HST images and on wide-field and narrow-field MUSE data cubes. In M87, dust filaments emerge from the nucleus and seemingly from jet knots. More dust than found in the literature is present, including a dust filament starting from the tip of the jet. The jet itself is fine-structured down to the HST resolution limit, so its relativistic character is in doubt. NGC 4696 shows similar dust properties. Better visible is here an optical continuum radiation from the dust which directly indicates cooling of a hot ISM in magnetic fields. All evidence support a scenario, where dust forms in situ as the final cooling product of a multi-phase magnetised ISM.

2023-01-12
11:15
Galactic archaeology with globular clusters in Andromeda
Ivan Cabrera-Ziri (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG

2022-12-15
11:15
Tracing the origin of galaxies beyond the Milky Way from limited observations
Sebastian Trujillo-Gomez (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
In this talk I will describe two complementary and novel approaches to understanding the assembly of galaxies beyond the MW in a cosmological context using observational tracers. In the first approach, we infer the origin of globular clusters from observables towards the goal of reconstructing the assembly histories of galaxies in upcoming wide-field surveys. Using the E-MOSAICS simulations to follow the formation and co-evolution of ca. 1000 galaxies and their star clusters, we explored the use of supervised machine learning to classify observed GCs into accreted and in-situ populations. Assessing the performance using a subset of the simulations and the known origin of the MW clusters, we obtain an accuracy of ca. 90% for 2/3 of the sample and successfully identify accreted debris buried deep within the Galaxy. In the second approach we study hundreds of high quality galaxy rotation curves to understand the impact of the large-scale environment on the structure of their host dark matter (DM) haloes. Galaxies in high density environments show a systematic shift in their DM density profile at large radii that is consistent with a relatively early assembly of their host haloes. The effect is manifest in the well known radial acceleration relation (RAR) as a slight downturn at the lowest accelerations for galaxies in dense environments. This environmental dependence can be understood in the context of assembly bias within the Lambda-CDM cosmological paradigm, implying that the RAR can provide useful constraints on the fundamental relation between dark and luminous matter.

2022-12-08
11:15
The role of the star clusters in the dynamical evolution of their planets
Francesco Maria Flammini Dotti (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The dynamical evolution of planetary systems is an important field that aims to explain how the majority of planets eventually obtain their actual architecture. In a star cluster, it may explain the diversity of their architectures due to their different encounter history. Moreover, there is a relative large abundance of free-floating planets in our galaxy. The star clusters may be a consistent source due to their ejected free- floating planets. In this talk, I will explain how the star cluster density and the presence of a cluster-centric intermediate-mass black hole eventually affects Solar-like systems (i.e., a complex planetary system) and single- planet systems respectively. Furthermore, I will introduce global rotation in the star cluster, and discuss its role in the ejection of both stars and free-floating planets. I will use NBODY6++GPU (a N-body code which performs simulations with a large number of particles, i.e., star clusters) and LonelyPlanets (another N-body code which performs simulations with a low number of particles, i.e., planetary systems). The results confirm that the density of the star cluster is one of the major characteristic in the ejection of planets from their host star, and the planetary architecture has also a predominant role. The intermediate-mass black hole enhances the ejection of planets from the planetary systems as well. Moreover, the ejection of both stars and planets from the star cluster is enhanced as well. Finally, the global rotation of star clusters diminishes the ejection of planets from their star cluster even at mid-slow rotational speeds.

2022-12-01
11:15
Mapping intrinsic Ly-alpha halos of high-redshift AGN with MUSE
Wuji Wang (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
High-redshift radio galaxies (HzRGs) have powerful AGN and are hosted by some of the most massive galaxies. They are surrounded by an enriched CGM allowing us to study feeding/feedback processes at and beyond Cosmic Noon. To understand the role of the CGM thoroughly, we observed a sample of eight HzRGs (2.92<z<4.51) with MUSE. In addition to the prominent Ly-alpha line, several UV emission lines (e.g. NV and CIV) are also captured and present absorption features. In our pilot study, we map the absorption- corrected halo of a z ca.4.5 redshift radio galaxy and find evidence that AGN feedback may play an important role in redistributing material and metals on CGM scale (Wang et al. 2021). In this talk, I will present the results of this technique applied to the full MUSE sample, i.e. mapping the intrinsic Ly-alpha emission halos of our HzRGs. The developed smoothing+tessellation technique for MUSE (or IFU) data allows us to capture faint and extended halo emission (7 have halos >100kpc) only limited by the data depth. HI Lya absorption features are often interpreted as radiative transfer effect. Seeing them in both hydrogen and metals, we argue that they are due to absorbing gas and must be considered in CGM analysis. In this way, we can decipher the morphological and kinematical information of the kpc-scale halos linking this with the feedback. Four of our HzRGs will be observed with JWST/NIRSpec, which will enable us to explore feedback from the vicinity of the central engine and study its propagation through the host galaxy and beyond into the CGM at larger scales.

2022-11-24
11:15
On the origin of multiple stellar populations in globular clusters
Silvia Martocchi (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Globular clusters (GCs) are very dense agglomerates of hundreds of thousands stars and they host some of the oldest known stars in our Universe. Their formation is one of the current outstanding problems in astronomy. GCs were traditionally thought to be simple stellar systems, assuming that all the stars within a given cluster were born at the same time and have the same chemical composition. However, already in the ‘70s, light-element abundance variations (e.g. He, C, N, Na, O) within GCs stars were discovered thanks to spectroscopy and photometry. A typical GC optical and near-UV color-magnitude diagram shows discrete multiple sequences at almost any evolutionary stage, which indicate the presence of multiple stellar populations (MPs). This has led to a new paradigm in our understanding of GC formation, which is intimately linked to the formation of stars and the assembly of galaxies. Indeed, GCs were once thought to form in special conditions, only present in the early Universe. The discovery of young GCs, forming in nearby galaxies, has opened a new window on the problem, allowing us to directly search for the origin of MPs. In this talk, I will present an overview of the MPs problem in GCs. I will report on the latest findings from both an observational and theoretical point of view, with a major accent on the discovery of such a phenomenon in young and intermediate age GCs. Finally, open questions and possible ways forward onto the MPs problem will be discussed.

2022-11-17
11:15
Dark matter and angular momentum in nearby disc galaxies
Pavel Mancera Pina (Leiden Observatory)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Dark matter and angular momentum are key parameters regulating the evolution of galaxies through cosmic time: they largely control their total mass, size, and morphology. During the last years, we have exploited exquisite observations and state-of-the-art analysis tools to robustly measure the motions of the cold gas in disc galaxies, which in turn allows us to infer their dark matter and angular momentum. This talk will be divided into three main parts. In the first part, I will focus on the startling dynamics of gas-rich ultra-diffuse galaxies (UDGs). UDGs are very peculiar systems: they have similar effective radii as big spirals like the Milky Way but about 1000 times fewer stars, making them very diffuse. By carefully modelling their gas kinematics, we found that our galaxies rotate much slower than other galaxies with similar baryonic mass, making them strong outliers of the baryonic Tully-Fisher relation. Moreover, our UDGs have baryon fractions as high as the cosmological average, and they appear to have dark matter distributions challenging to explain in the Cold Dark Matter model. In the second part of the talk, I will present some of the most detailed measurements of the baryonic specific angular momentum of nearby disc galaxies to date. We discovered a new relationship between the baryonic mass, baryonic specific angular momentum, and gas content; this is one of the tightest known scaling relations of disc galaxies. Finally, in the last part of the talk, I will show our recent and accurate determinations of the dark matter content in a sample of nearby galaxies. For the first time, we systematically accounted for the fact that their gaseous discs are not razor-thin but thick and flared, which allowed us to obtain some of the most detailed estimations of the dark matter content in nearby galaxies. We also revisited baryonic and dark matter scaling relations, finding evidence of feedback processes imprinting signatures on them.

2022-11-10
11:15
The decline of star formation: Exploring molecular gas, outflows and the early history of star formation
Caroline Bertemes (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
One of the key open questions in galaxy evolution today is how and why galaxies (almost) cease to form stars towards the end of their life cycle. The so-called quenching of star formation has been shown to be driven by a combination of reduced molecular gas fractions and low star-forming efficiencies. However, the detailed physical processes behind regulating these quantities, as well as the associated timescales, remain an active area of research. I will present new insights into galaxy quenching and the drivers of star formation fluctuations from various angles: Using the first data release of the MaNGA-ARO Survey of CO Targets (MASCOT; Wylezalek et al. 2022), I will investigate the more imminent impact of feedback processes onto molecular gas properties at low redshift in conjunction with ionised gas kinematics and the gas-phase metallicity distribution. Specifically, I will focus on weak outflows within the subset of active galaxies below the Main Sequence. Complementary, I will also report on the JWST Early Release Science program Q3D, which will zoom into strong outflows within luminous quasars at different redshifts (0.4 < z < 3). Finally, I will address to what extent the present-day star formation in galaxies may be traced back to their early histories of star formation on Hubble timescales. For this purpose, I will reconstruct the star formation histories of massive star-forming SDSS-IV MaNGA galaxies (z~0.04) via full spectro-photometric fitting with the novel stellar population synthesis code Bagpipes (Carnall et al., 2018).

2022-11-03
11:15
Molecular gas and star formation histories in void galaxies
Jesus Dominguez Gomez (University of Granada)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Galaxies in the Universe are not distributed uniformly, but rather in a web-like structure char- acterised by dense clusters, elongated filaments, sheetlike walls, and under-dense regions in between, called voids. Galaxies in voids are on average bluer, less massive, have later morphological types, and have higher star formation rates than galaxies in denser environments. Simulations show that different gas accretion modes dominate in each environment and the halo-to-stellar mass ratio is higher in void galaxies compared to galaxies in denser environments. This suggests that galaxies in voids evolve slower. Here I present results related to the molecular gas mass and star formation histories of galaxies in the voids, filaments, walls and clusters, trying to unveil the effect of the large-scale environment on galaxy evolution.

2022-10-27
11:15
HEALpix and MOC: Get into shape on the sky
Markus Demleitner (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
HEALpixes are a powerful method of dividing the sky into equiangular regions and cleverly enumerating them. In particular when doing statistical analyses over larger areas of the sky, there rarely is a good reason to invent any other scheme – indeed, if you have ever done non-trivial things with Gaia source_ids, you have worked with HEALPixes. On top of them, the Virtual Observatory has defined Multi-Order Coverage maps, or MOCs for short. These turn dealing with complicated, possibly non-connected areas on the sky from an extremely painful exercise into simple operations between integer sets. You can now use both of these things from ADQL (at least on some TAP servers), and you can use them to plot your results in TOPCAT (or pyVO). This talk will show you how.

2022-10-20
11:15
Classification of emission-line nebulae in the PHANGS-MUSE sample: a Bayesian approach
Enrico Congiu (University of Chile)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The PHANGS-MUSE survey offers us an unprecedented view of the ionized interstellar medium of nearby star-forming galaxies. The high spatial resolution (ca.50 pc) allows us to identify and isolate single emitting clouds, while the broad spectral range offers us the opportunity to study their properties in detail. However, a correct classification of the nebulae (e.g., HII regions, planetary nebulae, supernova remnants) is critical to studying them and their role in galaxy evolution. Many classification schemes have been developed in the literature in the last few decades. Still, none of them can take advantage of all the information provided by modern integral field spectrographs like MUSE at the VLT. In this talk, I will present the results of a new algorithm that aims to reliably and objectively classify nebulae using the Bayesian principle of the odds ratio to compare spectral and morphological properties of each region to models representing different classes of nebulae. This algorithm is applied to the nebulae identified in the PHANGS-MUSE survey to produce the most extensive catalog of classified ionized nebulae available in the literature.

2022-09-22
11:15
Astrometric Microlensing of Primordial Black Holes with Gaia
Himanshu Verma (Department of Physics Bombay, Mumbai)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The Gaia space telescope allows for unprecedented accuracy for astrometric measurements of stars in the Galaxy. In this work, we explore the sensitivity of Gaia to detect primordial black hole (PBH) dark matter through the distortions that PBHs would create in the apparent trajectory of background stars, an effect known as astrometric microlensing (AML). We present a novel calculation of the lensing probability, and we combine this with the existing publicly released Gaia eDR3 stellar catalog to predict the expected rate of AML events that Gaia will see. We also compute the expected distribution of a few event observables, which will be useful for reducing backgrounds. We argue that the astrophysical background rate of AML like events due to other sources is negligible (except possibly for very long duration events), and we use this to compute the potential exclusion that could be set on the parameter space of PBHs with a monochromatic mass function. We find that Gaia is sensitive to PBHs in the range of0.4 Msun to 5\times10^7 Msun, and has peak sensitivity to PBHs of \approx 10 Msun for which it can rule out as little as a fraction 3\times10^-4 of dark matter composed of PBHs. With this exquisite sensitivity, Gaia has the potential to rule out a PBH origin for the gravitational wave signals seen at LIGO. Our novel calculation of the lensing probability includes for the first time, the effect of intermediate duration lensing events, where the lensing event lasts for a few years, but for a period which is still shorter than the Gaia mission lifetime. The lower end of our predicted mass exclusion is especially sensitive to these type of lensing events. As and when time-series data for Gaia is released, our prediction of the lensing rate and event observable distributions will be useful to estimate the true exclusion/discovery of the PBH parameter space utilizing this data.

2022-09-05
12:40
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D (D)
ARI Institute Colloquium
D, D
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2022-07-28
11:15
Multiphase and multi-scale AGN feedback processes
Dominika Wylezalek (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
AGN feedback is now widely considered to be one of the main drivers in regulating the growth of massive galaxies. In my talk I will describe several efforts in our group to understand the power, reach and impact of AGN feedback processes. We find significant evidence for AGN feedback signatures even in low-luminosity AGN and we are now using molecular gas as a tracer to investigate if and how feedback may impact and quench galaxies at low redshift. At higher redshift, it appears that AGN-driven outflows can indeed suppress star formation in their hosts, consistent with the AGN having a â€˜negativeâ€™ impact on galaxy evolution. However, both star formation and quasar activity peak at z âˆ¼ 2-3 where AGN are expected to impact the build-up of stellar mass the most and I will present recent efforts in our group to characterise feedback processes in powerful AGN on CGM scales at and near Cosmic Noon. In particular, our team recently discovered a unique population of luminous high-z quasars (ERQs) with extreme outflow properties. At the same time, more and more exotic AGN populations with extreme signatures are being discovered at that redshift. These populations are ideal to obtain a census of the overall mass and energy budget of both outflow and infall/feeding from the CGM, an essential requirement to probe the detailed and full feedback loop. Finally, I will also introduce the JWST ERS Program â€Q3Dâ€ which will study the impact of three carefully selected luminous quasars on their hosts. Our program will serve as a pathfinder for JWST science investigations in IFU mode. Depending on JWSTâ€™s science schedule, I may show some of the very first JWST science images.

2022-07-21
11:15
First 3D radiation-hydrodynamic simulations of Wolf-Rayet winds and their effects on observations
Luka Poniatowski (Leuven)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Classical Wolf Rayet (WR) stars are hydrogen-free, late evolutionary stages of massive stars. These stars are direct supernova progenitors and undergo intense mass loss. As such, understanding dense and fast outflows of WR stars is crucial for understanding advanced stages of stellar evolution, the dynamical feedback of massive stars on their environments and the characterisation of the distribution of black hole masses. Given the complex optically thick, non-LTE environment, current insights on on WR outflows are usually made with a spherical 1D calculations. However, we know from observations and theoretical simulations that the winds of WR stars are clumped and thus non-spherical effects likely playing an important role to understand them. In my previous and current work, we are developing the first time-dependent, multi- dimensional, radiation-hydrodynamical models of the extended, optically thick atmospheres and winds of the classical WR stars. To make this feasible, we employed a hybrid opacity model using a combination of tabulated Rosseland mean opacities and enhanced line opacities expected within a supersonic flow, resulting in highly structured, turbulent flows. Performing radiative transfer calculations on the resulting hydrodynamic structure, we characterise some of the first conclusions for the observations of WR stars following from these 3D models.

2022-07-14
11:15
First 3D radiation-hydrodynamic simulations of Wolf-Rayet winds and their effects on observations
Luka Poniatowski (Leuven)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Classical Wolf Rayet (WR) stars are hydrogen-free, late evolutionary stages of massive stars. These stars are direct supernova progenitors and undergo intense mass loss. As such, understanding dense and fast outflows of WR stars is crucial for understanding advanced stages of stellar evolution, the dynamical feedback of massive stars on their environments and the characterisation of the distribution of black hole masses. Given the complex optically thick, non-LTE environment, current insights on on WR outflows are usually made with a spherical 1D calculations. However, we know from observations and theoretical simulations that the winds of WR stars are clumped and thus non-spherical effects likely playing an important role to understand them. In my previous and current work, we are developing the first time-dependent, multi- dimensional, radiation-hydrodynamical models of the extended, optically thick atmospheres and winds of the classical WR stars. To make this feasible, we employed a hybrid opacity model using a combination of tabulated Rosseland mean opacities and enhanced line opacities expected within a supersonic flow, resulting in highly structured, turbulent flows. Performing radiative transfer calculations on the resulting hydrodynamic structure, we characterise some of the first conclusions for the observations of WR stars following from these 3D models.

2022-07-14
11:15
The many catalogues of Gaia DR3
Stefan Jordan (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Every day about five papers are published using Gaia Data Release 2 or Early Data Release 3. Gaia Data Release 3, published on June 13, 2022, will be another source of high-precision data for the astronomical research. The number of stars with 6D phase space information (astrometry plus radial velocities) for stellar dynamical investigations has increased by almost a factor of five, and for the first time more than 100 million low-resolution Bp and Rp spectra become available, providing more detailed information compared to the previous broad-band photometry. Additionally, Gaia DR3 contains special catalogues for multiple stars systems, variable star, and asteroids. Nine performance verification papers, provided by the Gaia DPAC team, demonstrate how to efficiently use the new Gaia catalogue in many different areas of astrophysical research.

2022-07-07
11:15
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Joachim Wambsganss (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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2022-06-30
11:15
Building a microlensing look at the models of planet formation
Clement Ranc (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The Galactic Bulge Time Domain Survey of the NASA Nancy Grace Roman Space Telescope (launch in 2027) will detect >1000 planets with masses as low as Mars mass around G, K, M stars. The high resolution capability and high precision photometry of the telescope will also yield the mass measure- ments of âˆ¼700 such planets and their host stars during its prime mission. In this talk, I will present the physical concepts and observational techniques behind this space survey, using results already obtained from the ground, including from follow up observations of microlensing events that enable the mass-measurement of cold exoplanets on wide orbits (âˆ¼0.5-10 AU). Past microlensing surveys have measured the planet-to-host star mass-ratio function of the cold exoplanets in the Milky Way. When compared to predictions from planet population synthesis models, these observational results question some stages of the models of giant planet formation. After discussing these results, I will present a current program I am working on aiming at convert- ing the aforementioned â€˜mass-ratio functionâ€™ into a â€˜mass functionâ€™, which is expected to provide new insights and constraints on the abundance of cold exoplanets, its dependence to the host-star mass and location in the galaxy.

2022-06-23
11:15
Stripped stars, Wolf-Rayet stars, and the hunt for their black-hole remnants
Tomer Shenar (University of Amsterdam)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The ongoing LIGO/VIRGO gravitational-wave detections sparked an international â€œhuntâ€ for stellar-mass black holes in our own Cosmic neighborhood. Of special interest are black hole components in massive binaries (OB+BH). These systems offer key probes linking between their speculated progenitors â€“ Wolf Rayet binaries â€“ and their possible descendants: black hole mergers. Thousands of OB+BH binaries are expected to reside in the Milky Way and the Magellanic Clouds. However, only few are known, usually appearing as X-ray binaries. Multiple reports of OB+BH systems have emerged in the last years, but almost all have been refuted by following studies. Finding such systems will yield precious constraints on core-collapse physics and binary interaction at the upper-mass end. I will start the talk by discussing recent insights on the putative progenitors of black holes: Wolf-Rayet stars, including some open problems regarding their formation. I will continue by highlighting refuted black hole detections, and show how these led to the discovery of a new type of post-interaction binary systems. Finally, I will conclude with current ongoing efforts to find quiescent OB+BH systems using Gaia and spectroscopic techniques, including a few promising candidates.

2022-06-09
11:15
Perspectives on Massive Stars in the Local Universe and Beyond
Cormac Larkin (University of Groningen)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Stars account for the vast majority of the light we observe from the Milky Way and beyond. A single stellar population (SSP) is a group of stars formed at the same time and with the same initial composition. By combining superpositions of SSPs representing periods of star formation, we can model a wide range of galaxies. A long-standing issue when using empirical libraries to construct SSPs is non-arbitrary coverage of the HR-diagram, which imposes implicit age limits on SSP models that can be generated. The X-shooter Spectral Library (XSL) currently has poor coverage above Tef f âˆ¼ 10000K, limiting its applicability to young SSPs. I will discuss current efforts to populate a previously bare region of the XSL HR diagram and thus extend the lower age limit for XSL stellar population models by up to a factor âˆ¼10 for [Fe/H] = -1.2, and a factor âˆ¼2 for solar metallicity. Massive stars are particularly interesting at low metallicities as possible progenitors of gravitational wave sources, long-duration gamma-ray bursts and sources of cosmic reionization in the early Universe. The Small Magellanic Cloud (SMC) at Z âˆ¼ 0.2 Z provides the lowest metallicity environment where representative resolved populations of massive stars can be observed. I will discuss an ongoing project to find new high- significance candidate OB-type stars in the SMC. Using spectral energy distributions constructed from UV, optical and IR photometry, we have found hundreds of new candidate OB stars. Early results from spectro- scopic follow-up with Gemini-South are positive, thus potentially changing our view on the known massive stellar population in the SMC. Massive stars are also responsible for synthesising the majority of the short-lived radio-nuclides Al-26 and Fe-60 in the Galaxy. Upon decaying, they emit characteristic gamma rays which can be directly observed and mapped. While Al-26 can also be distributed by stellar winds, Fe-60 is only produced during a supernova. The comparison of both thus yields important constraints on the role and strength of stellar winds in evolved stars. I will talk about the importance of fully understanding the underlying nuclear reaction rates that create and destroy Al-26 in massive stars in order to gain a better perspective of the role of stellar winds and their yields.

2022-06-02
11:15
The effect of pre-processing on the stellar population content of early-type dwarf galaxies
Bahar Bidaran (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
According to the Î›CDM framework, clusters grow through accretion of individual galaxies and galaxies in groups, where they may be pre-processed. It thus becomes challenging for us to isolate and study the environmental effects solely due to the cluster host halo. Dwarf early-type galaxies (dEs) can provide us with statistically meaningful testbeds for investigating environmental effects thanks to their high number density and shallow potential well in the Local Universe. To study the imprint of pre-processing on the stellar population content of dEs, we analyzed a sample of nine Virgo dEs that, based on their distribution on the projected phase-space diagram and predictions from N-body simulations, have been accreted to Virgo as gravitationally bound members of a massive galaxy group, about 2-3 Gyr ago. We derived stellar population properties of these dEs (i.e., their age, [M/H] and [Î±/Fe]) using their MUSE spectra. We found that the dEs of our sample are considerably [Î±/Fe] enhanced and metal-poorer than equally-massive dEs of the Coma and Virgo clusters that have similar or even larger infall times. We also noted that 6 out of 9 dEs have experienced enhanced star formation (SF) at their accretion time or later, possibly due to shocks and thermal instabilities exerted by ram pressure inside Virgo. According to our results, the rest of our sample was accreted onto Virgo already quenched. Based on our results, we speculate that the short episode of star formation, occurred at infall into Virgo, might be responsible for the relatively high [Î±/Fe] ratios observed in these six dEs of our sample. Our sample of dEs exhibit on average, a negative metallicity gradient and a positive [Î±/Fe] gradient. Except for two dEs in our sample, the rest show a flat age gradient. These results are consistent with an inside-out formation scenario, and are in agreement with other observational and theoretical studies in the literature. In this talk, we take advantage of our results and discuss how pre-processing in previous host halos, and early processing in clusters may affect the physical properties of cluster dEs.

2022-05-19
11:15
Investigating the ionising particle environment of the Gaia camera
Edmund Serpell (Telespazio Deutschland, ESA/ESOC)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The Gaia spacecraft has been in operation near to the second sun-earth Lagrange point since early 2014. In this orbit it is exposed to a flux of charged particles with a wide range of energies originating from the sun and other energetic sources throughout the milky way galaxy. As these particles transit through the CCDs of the Gaia camera the evidence of their passage is recorded as ionisation tracks that are visible in a subset of the mission science data. Some of these primary particles undergo violent interactions with the spacecraft structure which results in the production of secondary ionising particles via nuclear fission. By analysing the information provided in the ionisation tracks it has been possible to identify particles from both of these particle populations and to study their energies.

2022-05-12
11:15
Tracing the onset of the feeding-feedback cycle in the nearby Narrow-line Seyfert 1 Galaxy Mrk 1044
Nico Winkel (MPIA)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The host galaxy conditions required for rapid black hole growth in active galactic nuclei (AGN) are poorly understood. Narrow-line Seyfert 1 (NLS1) galaxies often exhibit high accretion rates and are hypothesized to host prototypes of supermassive black holes at an early stage of evolution. In this talk, I will focus on Mrk 1044, the closest NLS1 that accretes above the Eddington limit. As part of the Close AGN Reference Survey we have observed Mrk 1044 with the adaptive optics assisted MUSE NFM-AO which allows us to resolve the circumnuclear star formation. We develop a kinematic model to trace ionized gas inflows from galaxy scales down to 100 pc from the nucleus. Although star formation appears to be the dominant ionization mechanism even in Mrk 1044â€™s very center, we detect a pc-scale high density outflow in the [O III] emission and LyÎ± absorption. Using a simple geometric model, I will discuss its origin and future impact on the host galaxy. This method has the potential to trace the onset of AGN feedback down to scales of the central engine, and has important implications for understanding black hole fuelling and feedback in its complexity.

2022-05-05
11:15
N-Body Simulations of Multiplanetary Systems in Star Clusters: The Effect of External Perturbations on the Dynamical Evolution of Planets
Katja Stock (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Stars predominantly form in groups or clusters, which, however, only dissolve completely after hundreds of millions of years. Within such stellar overdensities, stars interact gravitationally with each other. On timescales of the order of millions of years, close flybys of neighbouring stars occur, which can significantly alter the orbital architecture of the planetary systems that have formed around the stars in the cluster. In this talk, I present the results from my PhD thesis showing that gravitational perturbations from the birth environment can explain the large diversity in the orbital parameters of the observed exoplanet population. In particular, the simulation results show that about 1 - 2% of all planets adopt stable retrograde orbits due to external stellar perturbations or resulting interactions with other planets in the system. Furthermore, by taking into account tidal interactions between the host star and the planets, the formation of hot Jupiters can be observed in some systems as an indirect consequence of stellar encounters. Moreover, all simulated star cluster environments produce a significant percentage of unbound planets.

2022-04-28
11:15
Formation and Evolution of Supermassive Black Hole Triples in Cosmological Environment
Andreas Just (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Essentially all galaxies harbour a supermassive Black Hole (SMBH) in their centre. In standard cosmology galaxies grow by accretion of matter and by galaxy mergers. In the framework of a galaxy merger tree SMBHs grow by accretion and form binaries and multiple systems. The dynamical evolution of SMBHs in a galaxy merger remnant is covered by three processes: dynamical friction, 3-body encounters with stars and gravitational wave (GW) emission. In the first phase the SMBHs form quickly a binary and get harder down to parsec separation by dynamical friction. In the second phase energy is extracted by stars in the loss cone encountering the SMBH binary. In spherical systems the loss cone is quickly emptied and the evolution stalls. This so-called â€™Final parsec problemâ€™ was resolved with the help of numerical simulations by demonstrating that rotation or a weak triaxiality of the merger remnant is sufficient for a continuous supply of stars into the loss cone. In the final phase of GW emission energy loss becomes efficient on milli-parsec scale and leads to a quick merger of the SMBH binary. In recent years the efficiency of dynamical friction in the first phase was challenged. In galactic cores with a flat density profile dynamical friction can be lowered by orders of magnitude. Consequently, SMBHs are no longer merging on a Gyr timescale, but stall at kpc separation. With the next galaxy mergers, unstable multiple SMBH systems form. As a result the SMBH growth is reduced and free floating SMBHs should be created. Based on recent cosmological simulations, we investigate the formation of potential triple SMBH systems and study their evolution by high-resolution re-simulations.

2022-04-21
11:15
Detection of open clusters in Gaia data
Thomas Plewa (ITA)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Open Cluster are essential for the study of stellar evolution and an important tool for the under- standing of the dynamics and chemical composition of the Milky. Thanks to the high quality data provided by the Gaia mission there has been a surge in the number of open clusters in the recent years. These clusters are usually detected by unsupervised machine learning algorithms, so called clustering algorithms, that search for natural structures within data. In this talk we propose a different approach which is implemented in the Adaptive Matched Identifier of Clustered Objects (AMICO). AMICO is a matched filtering algorithm initially designed for the detection of galaxy clusters in wide field optical surveys and is currently in within the KiDS, J-PAS and Euclid collaborations. I show the performance of AMICO on Gaia DR2 and compare the results with other two state of the arts algorithms commonly used in the field. I will show how we can use AMICO to improve the quality and quantity of the cluster census in the Milky Way and how we can provide more insights into the structure of open clusters.

2022-02-17
11:15
B Supergiants: Atmospheres and Physical Properties
Matheus Bernini Peron (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
High-mass stars are very important to many areas of Astronomy. These objects deeply impact their surroundings through their powerful winds and their deaths as supernovae. Therefore,understanding the behavior of such stars is essential to understand their impacts on their hostgalaxies' properties and history.The aim of this research project is to analyze the atmospheres of B supergiants (BSGs, evolvedmassive stars) using the CMFGEN (Hillier & Miller 1998), a 1D, non-LTE atmosphere code — which is one of the state-of-the art tools used to analyze hot stars. The focus of the project is to investigate whether more recent models (e.g., the inclusion of x-rays, clumping, more recent atomic data) can better explain the optical and UV observed spectra of these stars, since previous studies failed to model several important UV lines (Crowther et al. 2006; Searle et al. 2008).As results we obtained (i) an overall improved agreement between BSGs observed and model spectra at the UV considering the effects of clumping and x-rays in the wind. Also we noticed (ii)important differences in their properties between hot (B1 – B0) and warm (B2 - B5) BSGs were also found, and it is in agreement with recent hydrodynamical simulations, such as Driessen et al.(2019). Beyond that, (iii), we have found a general trend of the CNO abundances for BSGs compatible with previous works in the literature and to the current high-mass stellar evolutionpredictions. However, (iv) despite a decrease in terminal velocity at the Bi-Stability Jump, we found no increase in mass-loss, instead, we have found a slightly decreasing trend towards later spectral types.

2022-02-10
11:15
Environmental dependence of the matter cycle from cloud evolution to star formation and feedback in 54 main sequence galaxies
Jaeyeon Kim (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The processes of star formation and feedback take place on the cloud scale (~100pc) within galaxies and play a major role in governing galaxy evolution. The properties of the clouds in which stars form are set by the large scale environment of their host galaxies, directly linking the initial conditions of star formation to galactic-scale properties. In turn, the energy, momentum,and mass deposited by stellar feedback drive the continuous evolution of the interstellar medium at large. Characterising the physical mechanisms regulating this multi-scale cycle is therefore crucial to understand the evolution of galaxies. By applying a new statistical method to the high-resolution CO and narrowband-Halpha imaging from the PHANGS survey, we systematically measure the evolutionary timeline from molecular clouds to exposed young stellar regions on the scales of giant molecular clouds across an unprecedented sample of 54 main sequence galaxies. We find that clouds live for about one dynamical time (8-30 Myr) and are efficiently dispersed by stellar feedback within 1.2-5.1 Myr after the star-forming region has become partially exposed. These ranges do not indicate uncertainties, but reflect physical galaxy-to-galaxy variation, implying an important dependence of these timescales on the local conditions, shaped by the galactic environment. The statistically representative PHANGS sample covers a large range of galaxy properties and morphologies, which allows us, for the first time, to quantitatively link galactic-scale environmental properties to the small-scale evolutionary cycle of molecular clouds, star-formation, and feedback. I will present the first census of these multi-scale trends. These results enable the characterisation of the physical mechanisms regulating cloud assembly, star formation, and cloud disruption, which eventually participate in driving galaxy evolution, as a function of the galactic environment.

2022-02-03
11:15
Dust Traffic Jams in Inclined Circumbinary Protoplanetary Discs
Hossam Aly (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Gas and dust in inclined orbits around binaries experience precession induced by the binary gravitational torque. The difference in precession between gas and dust alters the radial drift of weakly coupled dust and leads to the formation of dust traffic jams where the radial drift is minimised. I explore this new phenomenon using 3D SPH simulations and investigate its dependence on disc initial inclination and binary eccentricity. I will then present a new dust evolution model that takes the mutual gas and dust inclination into account and reproduce the SPH results, which provides a straightforward way to understanding dust traffic jams as a consequence of the drag torque exerted by the gas on the dust. Finally, I will present the results of radiative transfer post-processing of the hydro simulations and discuss possible observational implications of these dust traffic jams.

2022-01-27
11:15
The tale of the long uphill struggle of GBOT
Martin Altmann (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The highly praised ESA Gaia satellite mission has already provided the astronomic communitywith high quality astrometric, photometric and other data for almost 2 billion stars, and will continue to do sofor the next years. As time goes by, the precision of the astrometry increases with the number of measurementsand the time-span during which these are obtained, growing. Thus the correction of systematic effects in thedata, such as aberration need to be corrected to a point, where the conventional means do not suffice anymore.To accomplish this, a programme was conceived, to track the satellite with highly precise (20 mas) groundbased astrometry to deliver the required data for the optimisation of Gaia’s accuracy, called Ground BasedOptical Tracking (GBOT). This programme has faced many challenges and uncertainties, as well as set backs,but finally GBOT has come to the point, where its data are being included in the processing of the Gaiaastrometry, since 2020.This presentation will give an overview of the history of GBOT, and the steps taken to ensure final success,after many years of challenges. I will also report on a project searching for asteroids on the existing GBOTdata, which has lead to observations of about 42,000 objects, of which about 18,000 were previously unknown.

2022-01-20
11:15
Can velocity fields explain the Wolf-Rayet radius problem?
Roel Lefever (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Massive stars play a dominant role in the energetics of their host galaxies, primarily by their radiation-driven winds enriching their local stellar environments and by ionizing radiation. A subset of the massive stars, the Wolf-Rayet stars, which are direct progenitors of stellar-mass black holes, have particularly strong stellar winds. These winds are so powerful that they effectively push away the outer layers of the Wolf-Rayet star, obscuring it from sight. Hence, only the stellar wind can be observed from Earth. To infer stellar parameters, one needs to rely on a proper modelling of the winds of these stars. In this talk, I will show the deficiencies of the current wind modelling for Wolf-Rayet stars along with accompanying uncertainties on stellar parameters with solutions to construct more accurate models.

2022-01-13
11:15
More than a million stars - past, present, and future of direct N-body simulations
Rainer Spurzem (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Direct N-body computer simulations of the evolution of star clusters take into account all necessary - strong and soft - gravitational encounters between stars in the system. Since the times of von Hoerner and Wielen the ARI has been a place of cutting edge research in this topic. After a brief historic introduction I will introduce the current state of direct N-body simulation at the example of our mostly used code NBODY6++GPU, which is a research instrument having for our team the similar role as an observing instrument. Since a full simulation of a globular cluster over its lifetime of 12 billion years takes as many floating operations as the largest cosmological N-body runs, this is also partly a story of development of supercomputers and general purpose computing on graphics cards (GPU). The current record are models of star clusters with one million bodies (DRAGON simulations). Ideas are discussed how we will proceed in the future and why we need even more particles, e.g. to do proper modeling of nuclear star clusters. Depending on time one or two fields of current applications, done within our local and international team, will be shown and discussed: (i) evolution of black holes, their relativistic dynamics, and (ii) bound and free-floating planets in star clusters.

2021-12-16
11:15
The cloud-scale baryon cycle across the nearby galaxy population
Melanie Chevance (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The cycling of matter in galaxies between molecular clouds, stars and feedback is a major driverof galaxy evolution. However, it remains a major challenge to derive a theory of how galaxies turn their gasinto stars and how stellar feedback affects the subsequent star formation on the cloud scale, as a functionof the galactic environment. Star formation in galaxies is expected to be highly dependent on the galacticstructure and dynamics, because it results from a competition between mechanisms such as gravitationalcollapse, shear, spiral arm passages, cloud-cloud collisions, and feedback processes such as supernovae, stellarwinds, photoionization and radiation pressure. A statistically representative sample of galaxies is thereforeneeded to probe the wide range of conditions under which stars form. I will present the first systematiccharacterisation of the evolutionary timeline of the giant molecular cloud (GMC) lifecycle, star-formation andfeedback in the PHANGS sample of star-forming disc galaxies. I will show that GMC are short-lived (10-30Myr) and are dispersed after about one dynamical timescale by stellar feedback, between 1 and 5 Myr aftermassive stars emerge. Although the coupling efficiency of early feedback mechanisms such as photoionisationand stellar winds is limited to a few tens of percent, it is sufficient to disperse the parent molecular cloudprior to supernova explosions. This limits the integrated star formation efficiencies of GMCs to 2 to 10 percent. These findings reveal that star formation in galaxies is fast and inefficient, and is governed by cloud-scale, environmentally-dependent, dynamical processes. These measurements constitute a fundamental testfor numerical sub-grid recipes of star-formation and feedback in simulations of galaxy formation and evolution.

2021-12-09
11:15
An update of the Milky Way disk model and its application: Age-metallicity relation from 4 to 14 kpc based on the JJ model and APOGEE data
Kseniia Sysoliatina (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The semi-analytic Just-Jahreiss (JJ) model of the Galactic disk has been recently calibrated by us in the Solar neighborhood against the Gaia DR2 stars. We identified two star formation (SF) bursts that happened during the recent 4 Gyr. Now we present a global version of the JJ model applicable to a wide range of Galactocentric distances. This generalized JJ model includes exponential thin and thick disk and also atomic and molecular gas layers, as well as the flattened stellar halo and DM halo in the form of a cored isothermal sphere. The overall thickness of the thin disk is assumed constant at all radii, but flaring can be also modeled. The radial variation of the thin-disk star formation rate (SFR) reflects the expected inside-out disk growth scenario. Motivated by our findings for the Solar neighborhood, we allow the smooth power-law SFR to be modified by an arbitrary number of additional Gaussian peaks. Also, the vertical kinematics of the stellar populations associated with these episodes of the SF excess can differ from the kinematics prescribed by the age-velocity dispersion relation for the thin-disk populations of the same age. Using the observed metallicity distributions of the APOGEE Red Clump giants, we constrained the thin- and thick-disk age-metallicity relation for the distances 4 - 14 kpc. The public code of the JJ model is complemented by the two sets of isochrones, PARSEC and MIST. The generalized JJ model is a new stellar population synthesis tool that can be useful for a variety of tasks of Galactic archaeology, including the reconstruction of the Milky-Way disk formation history.

2021-12-02
11:15
Populations of satellite galaxies with the IllustrisTNG simulations: from galaxy clusters to the Local Group
Christoph Engler (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
I present populations of satellite galaxies in a Lambda-CDM context using the IllustrisTNG suite of cosmological magneto-hydrodynamical simulations. Utilising the entire range of IllustrisTNG allows for an unprecedented combination of statistical sample size and numerical resolution, resulting in mass ranges that cover multiple orders of magnitude for both host and satellites samples, as well as the first statistical sample of 198 high-resolution Milky Way-/Andromeda-like (MW/M31) hosts. I discuss the galaxy-halo connection for satellite and central galaxies across the mass spectrum in the stellar-to-halo mass relation as the most fundamental relationship of galaxy evolution in the cosmological standard model. I analyse the abundance of past and present-day satellite and subhalo populations around MW/M31-like hosts, find a remarkable degree of diversity, and put them into context with both observational surveys and previous simulations of similar systems. Their satellites become increasingly quenched towards smaller stellar masses as they lose their gas reservoirs more easily after infall. Thus, I not only give a detailed view on the evolution of satellite galaxies after infall and the environmental effects they experience but overcome one of the remaining challenges to the Lambda-CDM model: there is no missing satellites problem according to IllustrisTNG.

2021-11-25
11:15
Observational properties of O star- black hole binaries
Varsha Ramachandran (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
High Mass X-ray binaries (HMXB) with black hole companions represent a key intermediate stepin the binary black hole formation channel. Detailed knowledge of the stellar and wind parameters of thedonor is essential to understand the complex behavior of such systems. Our current understanding of wind ofmassive stars in black hole binaries is mostly based on Cygnus X-1 in our Galaxy and was not analyzed bymeans of sophisticated stellar atmosphere models. In this study, we carried out a detailed analysis of windsof doner star in M33 X-7 as well as a re-analysis of Cygnus X-1. M33 X-7 is the only known eclipsing blackhole binary with a very massive O supergiant donor and one of the most massive black holes known in anHMXB. In this talk, I will present a detailed spectroscopic analysis (Xray+UV+optical) of the massive donorduring different orbital phases. This sheds light on the stellar and wind parameters of the metal-poor donorstar. The observed properties are compared with detailed binary-evolution tracks to constrain the possibleformation channel and evolutionary fate of the system.

2021-11-18
11:15
The ROME/REA microlensing survey: Three years of Galactic bulge observations
Yiannis Tsapras (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The ROME/REA project (2017-2020) aimed to discover extrasolar planets by regularly monitoringmillions of stars in the Galactic bulge and looking for ongoing microlensing events. From April to Septembereach year, when the Galactic bulge was visible from the Southern hemisphere, the robotic telescopes of theLas Cumbres Observatory were used to observe a total area of about 4 square degrees in the sky in threedifferent bands. An automated process assessed ongoing microlensing events in real time for their sensitivityto planetary signals and additional observations were requested to characterize signals of particular scientificinterest. Our final catalog of stars contains more than 4 million individual sources. As we prepare for our firstpublic data release, I will present some of the results, talk about the data products we will soon be releasingand describe our current work and plans for the future.

2021-11-11
11:15
The Merger History of the Milky Way - What Gaia Revealed
Eva Grebel (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Cosmological models suggest that Milky Way-like galaxies are made up in part of stars that formed in situ and in part of stars that formed in other, smaller galaxies and that were subsequently accreted. Most of the more massive merger events should have occurred more than nine or ten billion years ago. In combination with massive ground-based photometric and spectroscopic surveys, Gaia is confirming and refining the cosmological picture. These data have uncovered numerous stellar tidal streams in our Galaxy, not all of which have known progenitors. Many come from disrupted dwarf galaxies, others from dissolving globular clusters - Gaia permits us to trace the detailed assembly history of our Galaxy, revealing the type of objects, their numbers, their properties, and the time of accretion. The most spectacular discovery is arguably that of the fairly massive dwarf galaxy Gaia-Enceladus or Gaia Sausage, which merged with the Milky Way about 10 Gyr ago. This event contributed many globular clusters and likely triggered the formation of the thick disk. In fact, Gaia data suggest that possibly half of our globular clusters come from merger events. Also, Gaia reveals the orbits of the surviving satellites, providing clues to their origins and future merger history.

2021-11-04
11:15
Stellar Winds in Turbulent Environments
Lachlan Lancaster (Princeton University)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Winds from massive stars have velocities of ~1000 km/s or more and produce hot, high-pressure gas when they shock. In the traditional, spherical model of these stellar winds, this high-pressure gas can act to quickly disperse the dense gas characteristic of regions where massive stars are born, acting to halt star formation. However, this classical theory is inconsistent with observations of wind-driven bubbles in the nearby universe and the observed high star formation efficiencies of super star clusters. I develop a new theoretical model for the expansion of stellar wind-driven bubbles that accounts for the turbulent structure of the surrounding gas. A key feature is the fractal nature of the hot bubble’s surface. The large area of this interface with surrounding denser gas strongly enhances energy losses from the hot interior, enabled by turbulent mixing and subsequent cooling at temperatures T ∼ 104–105 K, where radiation is maximally efficient. Due to this cooling, the solution is momentum-driven rather than energy driven, with resulting pressures in the shocked wind that are lower by up to a factor of 100. I explore the implications of such a theory and present a large suite of three-dimensional, hydrodynamical simulations that have been run to evaluate and test this theory. I also present simulations of self-consistently star-forming clouds where star formation is regulated solely by stellar wind feedback. These simulations allow us to test our theory in a more realistic context as well as track how wind material cools and collapses into subsequently formed stars.

2021-10-28
11:15
Changing the paradigm of globular cluster formation
Ivan Cabrera-Ziri (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Globular cluster formation is a major unsolved problem in astrophysics. The latest constraints tothe problem have come from puzzling abundance variations of light-elements among their stars. The pursuitto explain this longstanding problem using these chemical signatures has reinvigorated the study of globularclusters, and at the same time has challenged our understanding of nucleosynthesis and stellar evolution. Forthis talk, I will start with an overview of the challenges facing current models of globular cluster formation.Then I will present the steps being taken to build the next generation of globular cluster formation modelsand discuss how we can use the properties of globular cluster to trace the build-up of galaxies.

2021-10-21
11:15
Revealing the Milky Ways Dissolved Star Cluster Population with Dynamics, Chemistry, and HighDimension Analysis
Jeremy Webb (University of Toronto)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The Milky Way consists of a large number of star clusters, with over 150 being labelled as old,metal poor globular clusters and thousands being labelled as young, metal-rich open clusters. However thesenumbers are small compared to the total number of clusters that have ever existed in the Milky Way, as mosthave fully dissolved between their time of formation and the present day. Most of the constraints that havebeen placed on the Milky Ways dissolved star cluster population have been made by extrapolating how thestar cluster initial mass function, initial size function, and formation rate have evolved over time. In thistalk, I will introduce some more direct ways of studying dissolved star clusters that make use of internal starcluster dynamics, orbital dynamics, chemical tagging, and high dimension analysis. More specifically I willintroduce a new method for constraining progenitor cluster properties through deep observations of stellarstreams, demonstrate how stellar siblings can be identified through the combined use of orbital dynamics,chemical tagging, and discuss a new particle spray code that can be used with high dimensional analysis tofind extra-tidal stars.

2021-10-14
11:15
! ! ! Talk is cancelled ! ! !
Dr. Helge Todt (Potsdam University)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
! ! ! Talk is cancelled ! ! !

2021-07-22
11:15
MALT: Milky Way Archaeology using RR Lyrae and Type-II Cepheids
Hitesh Lala (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
RR Lyraes and Type-II Cepheids are old, low-mass variable stars and are invaluable distance indicators. We have created a comprehensive catalog of ~250,000 RR Lyraes and ~4000 Type-II & Anomalous Cepheids containing 7-D information (positions, proper motions, distances, radial velocities, metallicities). In the process, we have homogeneously combined 10 photometric and 6 spectroscopic surveys. Gaia EDR3 proper motions (and parallaxes) are available for ~95% of the sample. Using EDR3 parallaxes, along with the LMC population, we have derived new period-luminosity(-metallicity) and period-Wesenheit(-Metallicity) relationships in the Gaia DR2 & EDR3 GBPRP, VI, griz, JHKs, and W12 bands for all the sub-types (RRab, RRc, BL Her, W Vir, pW Vir, RV Tau, ACep_F, ACep_1O). Simultaneously solving the P-L/W relationships, we have computed individual reddenings (~15% uncertainty) to our sample stars. For our RR Lyrae sample (both RRab and RRc sub-types), we have also obtained photometric metallicity estimates on a new homogeneous scale creating the largest sample of RR Lyrae iron-abundances. Putting it all together, we have computed precise distances (~5% uncertainty) for an unprecedented number of RR Lyraes and Type-II Cepheids resulting in the most complete catalog yet of these populations. Harnessing this catalog, we have discovered many new members of Galactic streams and over-densities. For a few well-studied substructures like Orphan, Pal 5, GD-1, Sagittarius, etc., the re-discovery of known members helps us present more precise distances and metallicities for them, while the new ones enable precise measurement of these parameters for the very first time for many of the less-studied substructures. Our catalog places the community in a good state to exploit the imminent arrival of numerous spectra from WEAVE, 4MOST, DESI, Gaia DR3/4 and ultimately facilitate not only the chemo-dynamically analysis but also the discovery of several other streams and over-densities. The homogeneity of our catalog can also be exploited to uniformly study the Milky Way halo at unrivaled depths.

2021-07-15
11:15
Decoding the light of stars: Stellar Atmospheres and the crucial role of hot, massive stars
Andreas Sander (Armagh Observatory + ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Almost everything we know about the Universe beyond our Earth stems from the light of stars. To decode the information that is imprinted in this light, we need to understand its origin in the outermost layers of the stars, the so-called "stellar atmosphere". Only a realistic physical model of these transition layers allows us to translate our observations into a proper understanding of stars. Consequently, stellar atmosphere models are a fundamental tool of modern astrophysics. In the massive star regime, Wolf-Rayet stars are a rare but important class of stars. A large fraction of Wolf-Rayet stars contains no or only a small amount of hydrogen, thereby providing a crucial benchmark for the late evolution of massive stars before collapsing into massive black holes. With mass-loss rates that are about ten times higher than those of O supergiants, just a few Wolf-Rayet stars are enough to easily outweigh the feedback of a whole population of OB stars. To properly understand and quantify the impact of Wolf-Rayet and other massive stars, their spectra need to be analyzed with the help of stellar atmosphere models. Located at the conjunction of theory and observation, my new Emmy Noether group at the ARI will investigate the parameters and impact of hot and massive stars with various approaches revolving around the use of current and next-generation stellar atmospheres. The seminar talk will provide an overview of the techniques and challenges of modern atmosphere models as well as outline the underlying concept for including a consistent hydrodynamic treatment. Focusing on the yet poorly understood winds of Wolf-Rayet stars, I will show recent results from a groundbreaking study of massive He-star atmosphere models, yielding the very first mass-loss recipe derived from first principles in this regime. With major consequences on e.g. maximum black hole masses or He II ionising fluxes, I will outline the importance of making progress in the field of stellar atmospheres and give a brief outlook on some of the core questions the new research group will address.

2021-07-08
11:15
Galaxies in voids: observational perspective
Evgeniya Egorova (Lomonosov Moscow State University, Sternberg Astronomical Institute)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Voids are low density regions in the Large Scale structure of the Universe. Due to the specific conditions, they are very well suited for the study of some fundamental questions: the search for predicted delayed galaxies, the study of galaxy evolution and star formation in extreme isolation, search for and study episodes of gas accretion onto galaxies, from companions or from gaseous filaments. According to the previous studies, voids contain the sizable fraction of gas-rich extremely metal-poor dwarf galaxies with metallicities of Z = (1/50-1/20) Z_solar. Their overall properties suggest their probable early stage of evolution. Therefore, they can be good real counterparts for predicted in recent simulations the Very Young Galaxies. Voids are considered as regions where the cold pristine gas accretion from large-scale filaments can still proceed. This may increase probability to find such unusual objects in the early stages of evolution. In order to get the most complete picture, it is worth studying void galaxies with various methods and approaches. In our analysis we use the optical spectral and photometrical data, ionized gas kinematics, as well as mapping of HI in the 21 cm line. I will present our study of processes affecting the evolution of void galaxies and the project on search for candidates to Very Young Galaxies. At the moment this project results in discovery of 10 new gas-rich XMP dwarfs with Z < 1/30 Z_solar.

2021-07-01
11:15
Constraining the Cosmic Baryon Cycle
Fabian Walter (Max Planck Institut für Astronomie, Heidelberg)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
New observations with ALMA have provided a census of the density of molecular gas in the cosmic volume defined by the Hubble Ultra-Deep Field. This molecular gas density shows an order of magnitude decrease as a function of redshift from z~2 to z=0. It follows, to first order, the dependence of the cosmic star formation rate density. This is remarkably different from the atomic gas phase that shows a rather flat redshift dependence. At low redshift, observations of the interstellar medium of nearby galaxies (in particular the HERACLES survey of molecular gas and the THINGS survey of atomic hydrogen) have demonstrated that the atomic gas is significantly more extended than the molecular gas (the latter being tightly correlated with star formation activity). A similar picture is also emerging in observations of high-redshift galaxies. Assuming a simple galaxy model based on these findings, and using other measurements from the literature, the ALMA Hubble Ultra-Deep Field data are used to put observational constraints on the gas (net) accretion flows in galaxies. These gas flows are needed to explain the build-up of the stellar mass in galaxies, and are further compared to cosmological galaxy formation simulations.

2021-06-24
11:15
Spatially mapping the metal-enriched absorbing CGM of a massive galaxy at z~4.5
Wuji Wang (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract

2021-06-17
11:15
Superbubbles, stellar feedback and triggering of star formation in nearby galaxies
Oleg Egorov (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Stellar feedback plays an important role in regulation of the structure, kinematics and chemical abundance of interstellar medium. Multiple stellar winds and supernovae explosions create large holes and superbubbles in the ISM with sizes varying from several pc to several kpc, which were detected in many nearby galaxies. The interaction of the superbubbles might even trigger the new episode of star formation, while an intensive starburst could cause a development of the galaxy-wide outflows. These effects are especially important in the dwarf irregular galaxies - the feedback-driven structures can grow to larger sizes than and survive longer thanks to lack of spiral waves and the thick gaseous disks. In my talk I will focus on the multiwavelength analysis of the interplay between massive stars and ISM in nearby galaxies. In particular, I will overview the results of our observations of the ionized gas in star-forming regions of nearby dwarf galaxies performed with the high spectral resolution Fabry-Perot interferometer (FPI). I will also introduce the SIGMA-FPI archive which contain the FPI data cubes in Halpha line and results of their analysis for about 80 nearby dwarf galaxies.

2021-06-10
11:15
The Elephant in the Bathtub: When the physics of star formation regulate the baryon cycle of galaxies
Jindra Gensior (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Recent observational studies point towards a decreasing gas fraction and a low star formation efficiency (SFE) as the key drivers for star formation quenching in galaxies. However, what drives this SFE decrease, especially in early-type galaxies, is unclear. One proposed mechanism, morphological quenching, suggests that the global galactic environment can affect the gas dynamics such that star formation is heavily suppressed. I will present a suite of hydrodynamic simulations of isolated galaxies, which includes a new sub-grid star formation model capturing the influence of galactic dynamics on the SFE via the virial parameter of the gas. The parameter space spanned by the simulations ranges from disc galaxies to spheroids, with initial gas fractions between 1 and 20%. This enables a detailed exploration of how differences in the gravitational potential/morphology change the properties of the gas and the SFE, as well as how it interlinks with the gas fraction. I show that the shear generated by the deep gravitational potential of bulges can suppress star formation in the central regions of galaxies by altering the dynamical state of the gas and rendering it supervirial. This dynamical suppression of star formation is enhanced at higher stellar surface densities and lower gas fractions. Furthermore, I demonstrate that the resultant ISM structure (gravitational stability, resulting clumpiness, velocity dispersion) is also strongly affected by gas fraction and morphology. Together, these physical mechanisms drive the simulated spheroid-dominated galaxies off the main sequence, into the quenched population of galaxies, demonstrating that the physics of star formation can limit and regulate the baryon cycle at low redshifts and high galaxy masses.

2021-05-27
11:15
Purveyors of fine halos -- globular clusters as halo builders
Andreas Koch (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Globular clusters (GCs) play an important role in building up the Galactic halo. Former GC stars are readily identified by their chemical abundance signatures and from large (SDSS) samples we determined that 11% of the halo once originated in disrupted GCs. Adding the kinematical dimensions from Gaia we can go further and detect stars around present-day GCs, adding emphasis to the "stream" aspect and the halo-GC connection.

2021-05-20
11:15
SDSS-V: Pioneering Panoptic Spectroscopy
Kathryn Kreckel (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
SDSS-V will be an all-sky, multi-epoch spectroscopic survey of over six million objects. This will be carried out in three complementary programs. With the Milky Way Mapper (MWM) it is designed to decode the history of the Milky Way, trace the emergence of the chemical elements, reveal the inner workings of stars, and investigate the origin of planets. With the Local Volume Mapper (LVM) it will also create an integral-field spectroscopic map of the gas in the Galaxy and the Local Group that is 1,000x larger than the current state of the art and at high enough spatial resolution to reveal the self-regulation mechanisms of galactic ecosystems. Finally, SDSS-V will pioneer systematic, spectroscopic monitoring across the whole sky, revealing changes on timescales from 20 minutes to 20 years. With the Black Hole Mapper (BHM) it will thus track the flickers, flares, and radical transformations of the most luminous persistent objects in the universe: massive black holes growing at the centers of galaxies. Science observations for the MWM and BHM have begun already at the end of 2020, and LVM very recently had a groundbreaking for the construction of their new facility at LCO. As Heidelberg University is/will soon be a full institutional member, ZAH members are all welcome to become involved and contribute to any of the three projects in this very exciting early stage as we move from survey design into operations. In this talk, I will give an overview of all projects and science cases.

2021-05-06
11:15
Evolution of single and binary stars in gravitational N-body simulations
Albrecht Kamlah (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Direct N-body simulations are the only simulation method capable of properly resolving dense star clusters from the scales between close interacting stars in a binary all the way up to their interaction with distant halo stars. Nevertheless, they are frequently complemented with approximate Monte-Carlo methods, because these are much quicker computationally. Recently, there have been a number of code advances concerning the stellar evolution. This is crucial, as the evolution of individual stars has a huge effect on the overall dynamical evolution of a star cluster and thus amongst others the formation of gravitational wave emitting sources that may be detectable with LIGO and VIRGO. In this talk, I will highlight the state of stellar evolution routines in the direct N-body code NBODY6++GPU and the Henon-type Monte-Carlo code MOCCA and what can be expected from the next gravitational N-body simulations fof dense star clusters.

2021-04-29
11:15
Shaking Gaia: micro-meteorites, micro-clanks, micro-bubbles and micro-arcseconds
U. Bastian (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Gaia aims at producing micro-arcsecond-precision astrometry of a vast number of stars and other celestial sources. This involves the task of reconstructing and understanding the rotational motions of the free-floating Gaia spacecraft to micro-arcsecond-precision. The Gaia "First Look" group of ARI is in charge of the root steps of this task, plus a daily in-depth verification of the instrument health on board and the scientific quality of the data coming to the ground. Sounds boring? But it isn't! I will describe and explain a few of the adventures of Gaia's "First Look Scientists" at ARI, including a very recent one.

2021-04-22
11:15
Gas, stars, and globular clusters in the local Universe as tracers of galaxy formation and evolution, and the nature dark matter
Sebastian Trujillo (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
In this talk I will discuss how the stellar, globular cluster (GC), and gas components of galaxies allow us to trace the assembly of galaxies and their dark matter (DM) haloes, and how these constrain the complex physics of galaxy formation. I will use examples from three studies: In the first, I will describe how studying the phase-space distribution of the MW GC system using Gaia in the context of the E-MOSAICS simulations provides a detailed quantitative picture of the formation of the Galaxy. In the second example, I will show how the unusual GC populations in galaxies like the infamous NGC1052-DF2 and DF4 can be used to rewind the clock and obtain a snapshot of their galactic progenitors at cosmic noon. A simple model of star cluster formation points to an extremely dense birth environment and strong structural evolution, providing clues of the effect of clustered star formation on galaxy evolution. In the last part I will describe a follow-up study of the impact of clustered star formation on galaxy structure that provides clues about the origin of ultra-diffuse galaxies (UDGs), which are difficult to explain in the current paradigm of galaxy formation. I will show how anchoring an analytical model on galaxy scaling relations and numerical simulations predicts the emergence of UDGs that lack DM driven by clustered feedback from young GCs.

2021-04-15
11:15
Scatter in the star-forming Main Sequence: A remnant of long-term variations in stellar mass growth?
Caroline Bertemes (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
About 90% of the stellar build-up in galaxies occurs gradually on the main sequence, with the tightness of this relation (at the level of ~ 0.3 dex) being commonly interpreted as a consequence of the self-regulative nature of galaxies. However, within this framework, it is currently still unclear whether there are multiple pathways of stellar mass growth. In other words, does the observed scatter stem from systematic long-term differences in the star formation histories of galaxies that differ in their sSFR today (set e.g. by variations in halo assembly)? Or can the spread simply be attributed to short-term stochastic fluctuations in the growth rates of galaxies (traced back to e.g. variations in gas inflow, minor mergers, "breathing" cycles consisting of star-bursting episodes followed by a suppression due to feedback)? In addition to defining the timescales on which the main processes regulating star formation operate, these scenarios are also indirectly related to the end of the lifecycle of galaxies. By discriminating between a predominantly smooth or bursty evolution of galaxies, quenching may be interpreted as a natural progression of continuously declining star formations histories (slow quenching) or a disruptive process (fast quenching). I will present new insights into the the star formation histories of massive star-forming SDSS-IV MaNGA galaxies, as reconstructed via full spectro-photometric fitting with the novel stellar population synthesis code Bagpipes (Carnall et al., 2018).

2021-02-25
11:15
Formation and evolution of supermassive black holes.
Melanie Habouzit (MPIA and LSW)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Supermassive black holes of million solar mass and above are commonly hosted by massive galaxies, but are also present in local dwarf galaxies. Black holes are a fundamental component of galaxies and galaxy evolution, but their origin is still far from being understood. Large-scale cosmological simulations are crucial to understand BH growth and their interplay with their host galaxies. We recently compared the black hole population of six of these large-scale cosmological simulations (Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA) and I will review how the simulation sub-grid models affect the build-up of the BH population and their correlations with galaxies properties. The next two decades will be dedicated to the exploration of the high-redshift Universe with upcoming space missions such as LynX, Athena, JWST, WFIRST, and LISA. I will present how we can use cosmological simulations to prepare these missions and maximize their scientific return.

2021-02-18
11:15
Breaching the barrier: dynamical formation of the first intermediate-mass black hole discovered by LIGO-Virgo
Manuel Arca Sedda (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
On September 2nd 2020, the LIGO-Virgo collaboration announced the detection of GW190521, a gravitational wave source associated with the merger of two black holes (BHs), 66 and 85 Mo masses, which left behind an intermediate-mass black hole (IMBH) with a mass of 142 Mo. This binary merger is peculiar because its primary mass falls in the so-called upper mass-gap, a region of stellar masses where modern stellar evolution predicts the absence of remnants, and the final remnant represents the first specimen of a "light" IMBH. In this seminar, I will describe a novel channel suitable to explain the properties of GW190521, namely a sequence of three mergers among stellar mass black holes. We discovered serendipitously such a process in a high-resolution N-body model of a dense star cluster. We combine these simulations with an analysis based on numerical relativity fitting formulae and on observed properties of globular, young, and nuclear clusters, to show that if GW190521 originated via such a mechanism its observation gives us insights on the distribution of stellar BH natal spins and on the environment that harboured such a system, most likely a dense and young star cluster.

2021-02-11
11:15
Ready for the next gravitational million-body simulations: Evolution of single and binary stars in NBODY6++GPU and MOCCA
Albrecht Kamlah (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Direct N-body simulations are the only simulation method capable of properly resolving dense star clusters from the scales between close interacting stars in a binary all the way up to their interaction with distant halo stars. Nevertheless, they are frequently complemented with approximate Monte-Carlo methods, because these are much quicker computationally. Recently, there have been a number of code advances concerning the stellar evolution. This is crucial, as the evolution of individual stars has a huge effect on the overall dynamical evolution of a star cluster and thus amongst others the formation of gravitational wave emitting sources that may be detectable with LIGO and VIRGO. In this talk, I will highlight the state of stellar evolution routines in the direct N-body code NBODY6++GPU and the Henon-type Monte-Carlo code MOCCA by comparing the results of small star cluster simulations performed with both. The results of these are important groundwork to ensure that the upcoming million-body simulations of globular and nuclear star clusters are astrophysically up-to-date.

2021-02-04
11:15
Microlensing events all over the sky - the promise of large-scale surveys
Markus Hundertmark (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Modern wide-field surveys will provide us with alert streams generating millions of alerts every night and by that providing a comprehensive picture of the variable sky. Traditionally, microlensing events have been found in the most crowded place in the Milky Way. Thanks to the footprint, microlensing events in the disk will play an increasingly important role. The opportunities opening up by these surveys will lead to more discoveries and the characterization of extrasolar planets (bound and unbound), black holes and insights into the binarity of stars and brown dwarfs. The most audacious surveys will certainly provide the best alert stream but some of the aforementioned science cases still require automated follow-up observations. The "fire-hose of alerts" demands cloud-based tools to trigger heterogeneous follow-up observations with a homogeneous software package - the TOM (Target and Observation Manager) Toolkit. We will show early results from the most recent observing programs running at Las Cumbres Observatory (LCO) global network using a TOM and how that can be applied to the surveys of the future.

2021-01-28
11:15
The role of star formation environment for planet formation
Andrew Winter (ITA)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The diverse architectures of discovered exoplanetary systems have provoked an equally diverse range of explanations of the processes governing their formation and evolution. However, the star formation environment is a factor that is frequently overlooked in studies that aim to synthesise the observed planet properties. I review the growing evidence suggesting that environmental feedback plays a significant role during and after planet formation. Two mechanisms that may be particularly influential are external photoevaporation due to irradiation of the protoplanetary disc by neighbouring OB stars, and subsequently star-star encounters that can generate instability in the mature planetary system. I discuss observational constraints and examples of these mechanisms caught in the act, and how they might alter a system from that which forms ?in isolation'. Finally, I contextualise the Solar System in terms of possible environmental sculpting.

2021-01-21
11:15
Refining the picture of galaxy regulation with observations & theory
Benjamin Keller (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Stellar feedback connects the smallest, densest scales of the galaxy to the largest, most diffuse components of the galaxy halo. The detailed, fractal structure of the interstellar medium (ISM) and the evolution of stars sets the local coupling efficiency of energy from stellar winds, radiation and supernovae. This feedback can then drive outflows from most galaxies, removing mass and metals to the diffuse, hot circumgalactic medium. This low-density halo can act as a reservoir for material, slowly allowing it to re-accrete and fuel ongoing star formation. This in turn influences the formation of new molecular clouds within the ISM, connecting the process of star formation to all scales of the galaxy's gas content. In this talk, I will present recent results that have helped to clarify the details as to how the details of these processes can impact the overall life of galaxies, and how we can use new observational and statistical methods to constrain these details.

2021-01-14
11:15
The power of asteroseismology: the internal structure of stars
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Over the past decade the space missions CoRoT, Kepler and TESS have revolutionised the field of asteroseismology ? the study of the internal structures of stars through their global intrinsic oscillations. A particular steep increase in our knowledge has been possible in stars cooler than ~ 6700 K, which exhibit convection in their outer layers. Oscillations are excited in these turbulent layers that allow us to probe large parts of the stars. In this talk I will present recent results of asteroseismic inferences of the stellar structure of these cool stars.

2020-12-17
11:15
Science Forgeries, Fraud, Fake Science Across Scientific Disciplines and Their Consequences
Eva Grebel (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The above was the subject of a Marsilius Project at Heidelberg University undertaken jointly by the art historian Prof. Henry Keazor and myself, triggered by my experiences in the DFG commission for investigations of scientific misconduct and his work on exposing art forgeries. I will present a summary of our findings on the different types of such misconduct across different disciplines, discuss the motivation of the perpetrators and ways of preventing misconduct, as well as talk about the consequences for science, scientific credibility, political and economic decisions, and public perception in general.

2020-12-10
11:15
Gaia Early Data Release 3
Stefan Jordan (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
On December 3, 2020, the first (early) part of Gaia’s third star catalogue will be published. Based on 34 months of Gaia observations it contains a significant increase in the number of entries compared to Gaia DR2: position measurements of more than 1.8 billion stars as well as proper motions, parallaxes, and broad-band photometry for about 1.4 billion stars. Additionally, both the precision and accuracy of the astrometric data has significantly improved, especially the determination of proper motions. Together with the release of the star catalogue, four papers are published, impressively demonstrating the scientific potential of the Gaia EDR3 catalogue.

2020-12-03
11:15
No Talk in ARI Colloquium on December 3, 2020, due to Gaia Data Release EDR3. The originally scheduled talk by Peter Berczik will take place in Rainer Spurzem's Tea Seminar on January 19, 2021.
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ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Astronomisches Rechen-Institut, Mönchhofstraße 12-14

2020-11-26
11:15
The strongest magnets in the Universe
Fabian Schneider (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Magnetic fields are ubiquitous in the Universe. The Sun’s magnetic field drives the solar wind and causes solar flares and other energetic surface phenomena that profoundly affect space weather here on Earth. The first magnetic field in a star other than the Sun was detected in 1947 in the star 78 Vir. Today, we know that about 10% of these intermediate-mass and high-mass stars have strong, large-scale surface magnetic fields whose origin has remained a major mystery till today. In this talk, I will present the first 3D magneto-hydrodynamical simulations of the coalescence of two massive main-sequence stars and 1D stellar evolution computations of the subsequent evolution of the merger product that can explain the origin of strong magnetic fields in massive stars. I will argue that such magnetic massive stars are promising progenitors of those neutron stars that host the strongest magnetic fields in the Universe, so-called magnetars, and that may give rise to some of the enigmatic fast radio bursts.

2020-11-19
11:15
Studying quasar microlensing with robots
Robert Schmidt (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Gravitationally lensed quasars are mirages where a background quasar is observed several times due to the light deflection by a foreground galaxy. Microlensing along the individual light paths causes variability of the quasar images, which lets us draw conclusions on the microlensing objects and the quasar engine. I sum up the state of the art and describe our monitoring program which uses the LCOGT robotic telescopes.

2020-11-12
11:15
CAPTURE OF INTERSTELLAR ASTEROIDS AND COMETS
Walter Dehnen (ARI)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
The recent by fly-bys of Oumuamua and Borisov suggest a large space density of such interstellar asteroids and comets. I investigate the possibility of capturing such objects into the Solar system via a fly-by of Jupiter or Saturn, presenting analytical arguments and estimates as well as results of various numerical simulations. The most likely captures occur for an incoming speed of around 0.6 km/s and populate orbits akin to those occupied by long-period comets. We estimate that the Solar system may contain around 10000 captured Oumuamua-like asteroids and 100 captured comets.

2020-11-05
11:15
A Light in the Dark - Massive Star Birth Through Cosmic Time
Jonathan Tan (Chalmers Uni & University of Virginia)
ARI Institute Colloquium
ARI, Moenchhofstrasse 12-14, Seminarraum 1, 1.OG
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Abstract
Massive stars have played a dominant role in shaping our universe since its earliest times, but there is still no consensus on the mechanism by which they form. I review the physics that is important for massive star formation and the connection this process may have with star cluster formation. I then focus on a particular theoretical model, Turbulent Core Accretion, which assumes the initial conditions are massive, turbulent, magnetized cores of gas and dust that are reasonably close to virial equilibrium. Our group has been exploring this scenario via analytic models and numerical simulations of the physics and chemistry of the interstellar medium, ranging from the earliest pre-stellar core phase to protostellar cores being impacted by strong self-feedback. Crucially, these models can now be tested in detail with ALMA, SOFIA and other facilities, and I present the latest results from multiple projects that are zooming in to massive star birth in the darkest shadows of giant molecular clouds. Extension of this work has the potential to also determine how the full stellar initial mass function is established across different Galactic environments. Finally, I discuss an application of massive star formation theory to the early universe: how massive were the first stars and could they have been the progenitors of supermassive black holes?

2020-07-30
11:15
The Density Gradient Inside Molecular-Gas Clumps as a Booster of their Star Formation Activity
Genevieve Parmentier (ARI)
ARI Institute Colloquium
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Abstract
The presence of a gas volume-density gradient inside star-forming regions allow them to raise their star formation rate (SFR) compared to what they would experience if their gas was of uniform density. I define the “magnification factor 𝝵” as the ratio between the SFR of a centrally-concentrated clump and the SFR that this clump would experience should its gas be uniformly distributed. I show that magnification factors higher than 10 are achieved by power-law gas density profiles with logarithmic slopes steeper than -3. Such steep density profiles describe well the densest regions of the nearby molecular clouds MonR2 and NGC 6334. Clumps with a high magnification factor form stars much faster than expected based on the mean free-fall time of their gas. Therefore, not only does a gas density gradient inflate the clump SFR, it also inflates the star formation efficiency per free-fall time that we measure. The diversity in measured star formation efficiencies per free-fall time thus partly reflects the diversity in star-forming region structures. I provide a method to quantify the contribution of the gas density gradient of a clump to its SFR, thus allowing one to estimate its magnification factor.

2020-07-23
11:15
SUPERMASSIVE BLACK HOLE FEEDING
Prof. Walter Dehnen (University of Leicester)
ARI Institute Colloquium
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Abstract
Supermassive black holes (SMBHs) form predominantly by accreting gas from an accretion disc, which is efficient only at radii < 0.001 pc. As this is well inside the region where the SMBHs gravity dominates, it remains unclear how material to be accreted can lose its angular momentum to reach these scales. Here, I will detail a possible solution to this problem, which is based on the cancellation of angular momenta of different inflows onto the innermost neighbourhood of the SMBH .

2020-07-16
11:15
Stellar mass excesses in radial gradients of massive, quiescent galaxies in HSC
Thomas Jackson (ARI)
ARI Institute Colloquium
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Abstract
We explore the stellar mass density and colour profiles of 118 low redshift, massive, quiescent, central galaxies in order to attempt to find hints of the minor merger activity postulated to be the driver behind the size growth of MASSIVE galaxies since high redshift. We use deep imaging data (down to ~26 mag) in 5 bands from the Subaru Hyper Suprime Cam survey combined with Voronoi binning to maximise signal to noise, and perform SED fitting to construct spatially well-resolved colour and stellar mass density radial profiles, stacking these profiles to utilise the full statistical power of the sample. We find slowly decreasing colour profiles, and an expected smooth, declining stellar mass density profile in the central regions of our sample (~3 Re), however excesses of stellar mass density in the outskirts in the form of bumps in the profile. By visually inspecting the data, we morphologically split the sample, finding that almost 45% of the sample display signs of minor merger activity, 45% display a diffuse stellar envelope, possibly indicative of previous minor merger activity and the rest display no activity. We find that the excesses of stellar mass in the outskirts are driven by those galaxies with clear signs of minor merger activity or a diffuse stellar halo, and that this material only contributes to a few % of the total stellar mass. We also apply similar techniques to the data as in previous studies, finding that these techniques smooth out the signatures of these interactions.

2020-07-09
11:15
How galactic dynamics and stellar feedback shape the giant molecular cloud population in galaxies
Sarah Jefferson (ARI)
ARI Institute Colloquium
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Abstract
Giant molecular clouds (GMCs) form the communication channel between the galactic environment and the sub-cloud physics of star formation and stellar feedback. Recent observations reveal galaxy-scale trends in GMC properties (e.g. surface densities, velocity dispersions, and turbulent pressures), demanding a systematic and predictive theory of the GMC lifecycle across the parameter space of observable star-forming environments. In this contribution, I will combine a simple analytic theory for the influence of galactic dynamics on GMC lifetimes/evolution with a statistical sample of ~60,000 numerically-simulated GMCs across three isolated disc galaxies in the moving-mesh code Arepo. The analytic predictions depend on just three sets of galactic-scale observables: the rotation curve, surface densities and velocity dispersions of the host galaxy. The numerical simulations incorporate the features required to accurately model the influence of stellar feedback on the ISM, including stochastic star formation, mechanical supernova feedback, pre-supernova feedback from HII regions, and ISM chemistry. Using this combined analytic/numerical framework, I will show (1) that stellar feedback truncates the GMC lifetime across a wide range of galactic environments, and (2) that the lifetimes and properties of GMCs are predicted to follow clear galactic-dynamical trends driven by gravitational instability, galactic shearing, radial orbital perturbations, and cloud-cloud collisions. These results reveal that the complex interplay between galactic dynamics and stellar feedback plays a crucial role in setting the course of GMC evolution, and therefore in determining the star formation rates of the galaxies they live in.

2020-07-02
11:15
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ARI Institute Colloquium
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Abstract
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2020-06-25
11:15
Observational signatures of AGN feedback across cosmic time
Dominika Wylezalek (ESO, ARI later this year)
ARI Institute Colloquium
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Abstract
AGN feedback is now widely considered to be one of the main drivers in regulating the growth of massive galaxies. In my talk I will describe several efforts to understand the power, reach and impact of AGN feedback processes. Using SDSS-IV MaNGA at low-z, we found that AGN signatures can be easily hidden in the integrated spectrum of a galaxy. At higher redshift, we find that outflows can indeed suppress star formation in their hosts, consistent with the AGN having a 'negative' impact. However, both star formation and quasar activity peak at z ~ 2-3 where AGN are expected to impact the build-up of stellar mass the most. Our team recently discovered a unique population of luminous high-z quasars (ERQs) with extreme outflow properties. ERQs are ideal to obtain a census of the overall mass and energy budget of both outflow and infall/feeding from the CGM, an essential requirement to probe the detailed and full feedback loop. I will also introduce the JWST ERS Program "Q3D" which we will study the impact of three carefully selected luminous quasars on their hosts. Our program will serve as a pathfinder for JWST science investigations in IFU mode.

2020-06-18
11:15
Improving Gaia's sky coverage in dense regions
Katja Weingrill (AIP Potsdam)
ARI Institute Colloquium
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Abstract
In dense regions of the sky ("crowded fields"), the standard Gaia operations scheme cannot cope with such an amount of overlapping stars. This has always been a known, unavoidable property of Gaia. However, around launch it was proposed to create special images from Gaia's Sky Mapper CCDs to fully cover crowded fields down to the detection limit, although at reduced measurement precision. During the development of Gaia, such images had been designed for technical purposes, but never been planned to be used for science. These images were recorded starting July 2014, i.e. right from the start of the scientific mission, and were just stored away for the time being. In early 2019 a small working group was set up within the scientific Gaia consortium (DPAC) to work on this topic. The talk will summarize the first steps and initial software solutions, the current status of the corresponding processing pipeline, as well as an outlook to the scientific results.

2020-06-04
11:15
Feedback from OB-stars on their parent cloud: Gas exhaustion rather than gas ejection
Elizabeth Watkins (ARI)
ARI Institute Colloquium
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Abstract
Stellar feedback from high-mass stars shapes the ISM of galaxies and thereby impacts the gas that will form future generations of stars. However, due to the difficulty in tracking the time evolution of individual molecular clouds, quantifying this is an observationally challenging task. In this talk, I present my recent study investigating the G316.75-00.00 high-mass star-forming ridge. G316.75 contains 18,900M? of H? gas, but only half of the ridge has actively formed O-stars. The second half remains quiescent. Such a situation provides a unique opportunity to observationally measure the impact of feedback by contrasting the properties of the two halves. I present archived Herschel and molecular line data of G316.75 and use this to calculate the balance between the kinetic and gravitational energy along the ridge. I show that despite the presence of 4 O-stars, the stability of the dense gas remains almost unchanged, demonstrating that stellar feedback cannot unbind the ridge. Using theoretical calculations, I show that such feedback inefficiency is expected given the large average density and elongated morphology of the ridge. However, due to the ionising photons emitted by the O-stars, further gas accretion onto the ridge is limited, leading to gas exhaustion rather than gas ejection.

2020-05-28
11:15
Formation and evolution of stellar clusters in a cosmological context
Marta Reina-Campos (ARI)
ARI Institute Colloquium
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Abstract
Stellar clusters are present in the local Universe in a variety of environments, from the current cluster formation sites in the disks of the the Antennae galaxies to the old GC population that mostly populates the halo of the Milky Way, implying that their evolution is tightly linked to that of their host galaxy. To understand the observed cluster populations, I will discuss the hypothesis that GCs are the relics of regular cluster formation at high redshift that survived until the present day. To quantitatively address this hypothesis, I will present the E-MOSAICS project, a suite of 25 present day Milky Way-mass cosmological zoom-in simulations that enables the self-consistent study of the formation and co-evolution of stellar clusters and their host galaxies through cosmic time. Using these simulations, I will explain how we can use GC populations to reconstruct the merger history of the host galaxy, with applications to the Milky Way.

2020-05-14
11:15
From satellites to intra-cluster light: the transformation of cluster galaxies
Yannick Bahé (Sternwarte Leiden University)
ARI Institute Colloquium
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Abstract
Galaxy clusters are the most massive virialized structures in the local Universe. They often contain a prominent “brightest cluster galaxy” (BCG), surrounded by a plethora of — mostly red, early-type — satellite galaxies and a faint halo of intra-cluster light (ICL). It is generally thought that these satellites are strongly affected by their special environment: for example, their star formation activity is suppressed, and tidal forces strip or even completely disrupted them into the BCG or ICL. In my talk, I will present predictions from the Hydrangea cluster simulations on the nature of these processes. In the simulations, new cluster satellites are efficiently stripped of their gas, with both resemblances and tensions compared to recent observations. In contrast, the stellar mass of simulated satellites is largely unaffected by tidal stripping in clusters, so that almost all satellites accreted since redshift one still exist at the present day. I will discuss what this implies for the origin and buildup of stars in brightest cluster galaxies and the intra-cluster light, and the impact of the cluster environment on galaxy morphology.

2020-05-07
11:15
Kraken reveals itself -- the merger history of the Milky Way reconstructed with cosmological simulations of the globular cluster population
Diederik Kruissen (ARI)
ARI Institute Colloquium
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Abstract
Globular clusters (GCs) formed when the Milky Way experienced a phase of rapid assembly. We use the wealth of information contained in the Galactic GC population to quantify the properties of the satellite galaxies from which the Milky Way assembled. To achieve this, we train an artificial neural network on the E-MOSAICS cosmological simulations of the co-formation and co-evolution of GCs and their host galaxies. The network uses the ages, metallicities, and orbital properties of GCs that formed in the same progenitor galaxies to predict the stellar masses and accretion redshifts of these progenitors. We apply the network to Galactic GCs associated with five progenitors: Gaia-Enceladus, the Helmi streams, Sequoia, Sagittarius, and the recently discovered, "low-energy" GCs, which we find to provide an excellent match to the predicted properties of the enigmatic galaxy “Kraken". The five galaxies cover a narrow stellar mass range (60-460 million solar masses), but widely different accretion redshifts (z=0.57-2.65). All accretion events represent minor mergers, but Kraken likely represents the most major merger ever experienced by the Milky Way, with a stellar mass ratio of 1:32 and a halo mass ratio of 1:7. Even though these progenitors likely were the Milky Way's most massive accretion events, they contributed a total mass of only a billion solar masses, similar to the stellar halo. This implies that the Milky Way grew its mass mostly by in-situ star formation. We conclude by organising these accretion events into the Milky Way's merger tree.

2020-04-30
11:15
Mapping the ionized ISM in nearby galaxies
Kathryn Kreckel (ARI)
ARI Institute Colloquium
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Abstract
Hence, direct measurements of stellar masses for single stars are important.

2020-04-23
11:15
Determine stellar masses with Gaia
Jonas Klueter (ARI)
ARI Institute Colloquium
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Abstract
The mass is the most substantial parameter of a star. It defines its temperature, surface gravity and evolution. Currently, relations concerning stellar mass are based on binary stars, where a direct mass measurement is possible.

2020-02-06
11:15
Shining light through hydrodynamics simulations
Maya Petkova (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
The presence of young stars can have a profound effect on the ongoing process of star formation in a molecular cloud. In particular, the light emitted by them can heat up or even ionise regions of the cloud and alter their dynamics and the future production of stars. Therefore, the numerical study of star formation could not be complete without accounting for the effects of radiative feedback. One of the accurate ways of modelling the propagation of light through a cloud is by using Monte Carlo radiative transfer (MCRT). I will present a new numerical scheme coupling MCRT and particle-based hydrodynamics to model ionising stellar feedback, and the expansion of the resulting H II region. The scheme has been thoroughly tested and further applied to a simulation of a star-forming cloud in the Central Molecular Zone (CMZ) of the Milky Way. Additionally, I will present synthetic emission maps of the simulated CMZ cloud prior to the predicted ionisation event, and compare them to observations.

2020-01-30
11:15
Unraveling the Hidden Substructure in Dwarf Elliptical Galaxies of the Virgo Cluster
Josefina Michea (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
Dwarf elliptical galaxies are commonly presumed to be simple systems: non star-forming and with a smooth spatial distribution of light. In this talk, we present a sample of 6 dwarf galaxies from the Virgo cluster which, at first sight, seem to be stereotypically passive and elliptical. However, through the application of a newly developed method, we are able to reveal spectacular spiral features which lay buried within the much brighter diffuse component of these galaxies. As a result, we find that for our sample of passive dwarf galaxies the spiral arms contribute ~2-14% of the total galaxy light within 2 effective radii. Next, we construct an hypothesis that explains the presence of these hidden features by performing high-resolution simulations of dwarf galaxies being thrown into a cluster potential. It is possible to reproduce the observed spiral features through the tidal-shocking of a thin, cold, and highly rotationally-supported stellar disk that plunges deep into the cluster core. This result implies that some passive dwarf galaxies may have a faint thin disk buried within a more luminous thick disk, the former only revealing its presence by forming spiral features after being subjected to tidal shocks.

2020-01-23
11:15
RR Lyrae stars as tracers of substructure and Galactic archaeology
Zdenek Prudil (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
The Galactic archeology is trying to understand the formation history of the Milky Way. To study the early Milky Way history one can use old population stars that carry information about their past in their current dynamical and chemical properties, which makes them invaluable in studies of the Milky Way formation history. In my research, I use old (< 10 Gyr) variable stars of the RR Lyrae class, which serve as standard candles and tracers of metallicity within the Local Group. In my talk, I will discuss their use for studying the northern stellar overdensity near the Small Magellanic Cloud (SMCNOD) together with their spatial and kinematical distribution in the Galactic bulge and Galactic disk. I will also address some of the open questions related to RR Lyrae variables, like the Oosterhoff dichotomy and the uncertainty in their mass due to the lack of RR Lyrae stars in binary systems.

2020-01-16
11:15
How Planetary Systems are Shaped by their Birthplace: Computer Modeling of Planetary Systems Embedded in Star Clusters
Katja Reichert (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
Young stars are almost exclusively found in groups, suggesting that most stars - and hence also most planets - form in stellar associations or clusters. Our Solar System is also thought to have formed in a clustered environment since isotopic ratios in meteorites indicate the occurrence of a supernova explosion close to the solar protoplanetary disk. Some star clusters dissolve within tens of millions of years but some also survive for hundreds of millions of years. In such environments, close stellar encounters are frequent, significantly affecting the dynamical architectures and stability of the planetary systems around those stars. We investigate how the initial orbital configuration of a planetary system affects its resilience against external perturbations due to other cluster members. By using a hybrid N-body code and current supercomputing hardware we simulate the dynamical evolution of planetary systems consisting of the four Solar System giant planets which orbit Sun-like stars in four different star cluster environments.

2020-01-14
11:15
The formation and evolution of planetary systems in stellar clusters
Simon Portegies Zwart (Sterrewacht Leiden)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1

2020-01-09
11:15
Dynamical Modelling and Evolution of Open Star Clusters
Andreas Just (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
Open star clusters (OCs) represent an important constituent of the disc in the Milky Way and in other galaxies. Each OC is made of a single/simple stellar popualation (SSP = one age, one metallicity) and provides a direct insight to the stellar initial mass function, stellar evolution, and the age-metallicity relation of the disc. The age distribution of the OC sample in the extended solar neighbourhood is well defined, while the cluster masses are very uncertain due to incompleteness, field contamination and the impact of unresolved binaries on the velocity dispersion. Gaia DR2 data promise a significant step forward concerning cluster membership leading to improved cluster properties including the observation of tidal tails. In order to determine the fraction of field stars formed in OCs detailed dynamical models are needed. I will discuss different aspects of OCs based on our analytical and numerical investigations including initial conditions, cluster lifetimes, cluster sizes and the structure of tidal tails. Our goal is to provide detailed models of individual OCs and to convert the observed cluster age-mass distribution to a cluster formation rate and cluster initial mass function.

2019-12-19
11:15
Dynamical interaction and energy exchange of stars with a Massive Black Hole binary
Branislav Avramov (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
Supermassive black hole binary systems (SMBHs) are the most promising sources of gravitational waves for the LISA mission. However, past theoretical models and numerical simulations showed that the hardening of the binary would stall at parsec-scale separations (Final Parsec Problem, FPP) and lead to merger timescales greater than the Hubble time. This project uses large-scale, high-precision N-body simulations of a SMBH merger, with a merger-induced triaxiality of the merger remnant. The simulations show an efficient, constant hardening rate of the SMBH binary, thus successfully resolving the FPP. In this talk, I will present a novel, fully GPU-parallelized hybrid integration approach which combines direct summation with the self-consistent field (SCF) method in order to efficiently represent the galaxy merger remnant potential, while conserving computational time. Using this approach, I study the interaction and subsequent ejection of interacting stars by the SMBH binary, focusing on analyzing the high energy and orbital element changes of these stars.

2019-12-12
11:15
Inferring Galactic parameters from chemical abundances: A multi-star approach
Jan Rybizki (MPIA)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
Despite the extensive amount of stellar elemental abundance data, the chemical evolution of stars is not that well understood. Given progress in the determination of stellar ages, the constraining power of chemical abundance data should be much better than it currently is. The main limitation from the galactic chemical evolution (GCE) modeller's point of view is our current understanding of elemental enrichment from nucleosynthetic processes, such as type 1a or core-collapse supernovae (SN). By using a simple one-zone GCE model within a sophisticated statistical framework, capable of exploiting the elemental abundances of hundreds of stars simultaneously, we are now able to score yield tables from literature by their ability to reproduce elemental abundance data. This is important to reduce the model uncertainty and also to make informed choices for costly hydrodynamical simulations that include GCE. Our results also suggest that this method is very sensitive to global GCE parameters, like the high-mass slope of the IMF. Since the project is open source and has many possible applications, feel free to contact me if you want to get involved.

2019-12-05
11:15
Arrival to Virgo: How would the cluster environment affect recent dE infallers during early stages of accretion?
Bahar Bidaran (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
It is observationally and theoretically well established that galaxy clusters grow through the accretion of isolated galaxies and galaxy groups. By accretion to a cluster, consecutive pericenter passages expose a cluster galaxy to strong environmental effects that transform its star formation activity, morphology, and perhaps its internal kinematics. Nevertheless, it is still not entirely clear how the galaxy environment transforms newly accreted satellites into early-type dwarf galaxies (dEs). If host halos such as clusters are in charge of any transformation in the kinematics of dE's progenitors, then at a fixed stellar mass range, do dEs with different infalling time to the host halo show different internal kinematics? This question can be addressed by members of a recently accreted group of galaxies to the Virgo cluster, discovered by Lisker et al. (2018). In this talk, I will present a kinematic analysis of 9 dEs in this particular group. I will compare their kinematics with other dEs in high and low-density environments, using the specific angular momentum (λR) parameter. I will show that recent dE infallers to a cluster such as Virgo exhibit (λR) profiles intermediate between ancient dE infallers and equally massive field galaxies. Moreover, I will discuss the role of pre-processing to explain the spread (λR) profiles we observed in these 9 dEs.

2019-11-28
11:15
faellt aus
Dominika Wylezalek (ESO (Garching))
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
Kolloquium faellt aus

2019-11-21
11:15
Next-generation stellar atmosphere models: From understanding spectra to creating a virtual laboratory
Andreas Sander (Armagh Observatory)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
Stellar atmosphere models are a fundamental tool in modern astrophysics with their typical applications ranging from analysing particular observations to predicting the appearance of unresolved stellar populations. With the inclusion of hydrodynamics, a new generation of atmospheres is emerging, which allows to consistently quantify the feedback from a given set of stellar parameters. The talk will provide an overview of the techniques and challenges of modern stellar atmospheres as well as outline the underlying ideas of next-generation models. Focusing on the yet poorly understood winds of H-depleted stars, I will show recent results indicating a substantial shift in our understanding of radiation-driven winds. With major consequences on e.g. maximum black hole masses or He II ionising fluxes, next-generation atmospheres likely will have a large-scale impact for various fields of astrophysics.

2019-11-14
11:15
The many flavors of starbursts
Florant Renaud (Lund Observatory)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
Interacting galaxies and mergers are key events in galaxy evolution, and are often associated with boosts of the star formation activity, in particular in the form of massive clusters. Observations still struggle to identify the main driver(s) of these starbursts and it is still not clear whether they are triggered by an increased mass of dense gas, and/or an increased star formation efficiencies. Knowing which process dominates in which physical conditions is key to extrapolate our understanding of star formation to the early Universe. In this talk, I will use simulations of interacting galaxies, at low and high redshift, to pin down the underlying physics of starbursts. I will show that complex combinations of physical processes are playing different roles along the evolution of a merger, but also across a given galaxy, and account for a diversity of efficiencies. I will demonstrate that, despite having comparable outcomes in the star formation rates, these different physical mechanisms leave different signatures, for instance in the CO emission and the formation of massive clusters.

2019-11-07
11:15
"Density is Destiny": How Environment Affects Galaxy Evolution
Anna Pasquali (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
Large photometric and spectroscopic surveys, together with large cosmological simulations, have shown that galaxy properties are controlled by both galaxy secular evolution and galaxy environment. The latter is typically parameterized in terms of local galaxy density, dark matter mass of galaxy groups/clusters, and galaxy hierarchy (centrals vs. satellites). Galaxy environment acts on galaxies in two main different ways: i) it can prolong the star-formation history and sustain the mass assembly history of centrals, the most massive galaxies in groups/clusters. ii) It also removes gas and stars from satellites with the result of progressively switching off their star formation activity. In this talk I will present the observational trends between satellites stellar properties and environment that we have derived as a function of galaxy stellar mass, group/cluster mass and phase space, and discuss the physical mechanisms responsible for them. I will also address how the current and future telescope, either ground- or space-based, can foster our understanding of environmental effects in the local and high-redshift Universe.

2019-10-31
11:15
3D Kinematics of Stellar Substructures in the Magellanic Cloud Periphery
Lara Cullinane (ANU)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
The Large and Small Magellanic Clouds (LMC/SMC), as two of the closest and most massive satellites of the Milky Way, have significant effects on the local Universe including the distribution of ultra-faint satellites and the orbits of tidal streams. Ongoing survey efforts ith the Dark Energy Camera have revealed a wealth of low-surface-brightness stellar substructures in the periphery of the Magellanic Clouds; characterising these structures will provide significant insight into the currently poorly-constrained masses and interaction history of the Clouds. In order to elucidate the properties of the structures, we have used 2df+AAOmega at the Anglo Australian Telescope to instigate a large-scale spectroscopic follow-up of stars across the Magellanic Cloud periphery. We are able to detect the kinematic signature of the Clouds up to projected distances of 23 degrees from the centre of the LMC. Combining our spectroscopically derived radial velocities with Gaia DR2 astrometry provides the first 3D kinematics for these regions. Our initial set of measurements, along a large substructure to the North of the LMC, reveal velocities near the extremity of the substructure and are significantly different from those expected from an extrapolation of the LMC rotation curve. Our ultimate aim is to use these 3D kinematics to assess dynamical models of the Magellanic Clouds; this will shed new light on the origin of the substructures and the evolution of the Magellanic/Milky Way system.

2019-10-24
11:15
The structure and formation of the Milky Way bulge : A view through double red clump
Dongwook Lim (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
The Galactic bulge is of importance to understand the formation of the Milky Way, as a massive and old stellar component. Studying the bulge is, however, hampered by the crowd of stars from various components, as well as extremely high and variable foreground extinction. The presence of two red clumps (RCs) observed in the color-magnitude diagram of the Milky Way bulge provides a great opportunity to study the structure and formation of the bulge. This double RC is widely accepted as evidence for a giant X-shaped structure originated from the bar instability. We suggested, however, a drastically different interpretation based on the multiple stellar population phenomenon as is observed in globular clusters (GCs), where the bright RC is from He and Na enhanced second-generation stars, while the faint RC is representing first-generation stars with normal abundances. Because these two scenarios imply very different pictures of the formation of the bulge, understanding the origin of the double RC is of crucial importance. As supporting evidence for multiple population scenario, here we report our discovery that the stars in the two RCs show a significant difference in CN-band strength. We also found that the red giant branch stars in the outer bulge are divided into two groups according to Na abundance in the [Na/Fe] - [Fe/H] plane. Since these chemical patterns and characteristics are only explained by stars originated in GCs, this is evidence that the double RC is due to the multiple population phenomenon. Furthermore, this result indicates that the outer bulge was mostly assembled from disrupted proto-GCs in the early history of the Milky Way.

2019-10-17
11:15
Probing the dawn of galaxy formation with helium - burning stars
Alessandro Savino (ARI)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstraße 12-14, Seminarraum 1
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Abstract
The study of nearby resolved galaxies is a source of transformational insights into the mechanisms that regulate galaxy formation and it currently provides our only empirical window on how star formation proceeds in the smallest dark matter haloes. To date, the most precise measurements of ancient star formation rely on the detection of the oldest main sequence stars. The challenging observational requirements for such a detection impose severe limitations on the quantity and quality of old star formation histories at our disposal. In this talk I will describe a new approach, aimed at expanding the capabilities of ancient star formation history measurements via modelling of th e old horizontal branch stars. I will present recent results on the ancient past of Local Group dwarf spheroidal galaxies revealing the complex formation history that they experienced. I will show that, thanks to its brightness, the horizontal branch has the potential to deliver very precis e star formation histories, for a large sample of galaxies and covering a cosmologically representative parameter space, constituting a treasure trove for the archaeological characterization of the Local Volume.

2019-07-25
11:15
Exploring the outskirts of the Milky Way with RR Lyrae stars
Gustavo Medina Toledo (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Pulsating variable stars play an important role in the study of the Galaxy's history since they are excellent tracing populations in uniquely identifiable evolutionary phases. RR Lyrae stars, in particular, are known Population II objects that can also be used as precise distance indicators. These stars have been recently used to reveal the early assembly history of the Milky Way, since the early phases of massive galaxy evolution are believed to have been dominated by the accretion of smaller galaxies containing old stars. Moreover, they are thought to be potential tracers of faint satellite systems hardly detectable by the traditional methods. In this talk I will present the results from an RR Lyrae search using data from the High cadence Transient Survey (HiTS), which was carried out with the Dark Energy Camera (DECam). HiTS is a campaign primarily aimed at detecting early supernovae explosions in real-time with the deep optical images DECam provide. However, the cadence and the strategy followed for the survey are well matched for RR Lyrae detection as well. Using data from HiTS we were able to detect new RR Lyrae stars out to 200 kpc from the Sun. I will discuss the results of the search for RR Lyrae stars using HiTS’ data, their connection with known or previously undiscovered satellite systems and halo substructures, as well as further implications of these findings.

2019-07-18
11:15
Exploring the free-floating planet population with gravitational microlensing
Przemek Mroz (Astronomical Observatory, University of Warsaw)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Theories of planet formation predict the existence of a population of free-floating planets that are not gravitationally tethered to any host star. Gravitational microlensing provides a unique tool for studying these objects. The first results of Sumi et al. (2011) claimed that Jupiter-mass free-floating planets are as common as main-sequence stars. However, these results disagree with censuses of substellar objects in young clusters and star-forming regions and with predictions of planet formation theories. I will present new results of the analysis of a ten times larger sample of microlensing events discovered by the OGLE-IV survey during the years 2010-2018, which shed new light on the population of free-floating planets. I will also discuss prospects for detecting free-floating planets with the future missions, like Euclid and WFIRST.

2019-07-11
11:15
Double stars are trouble stars: Close pairs in Gaia DR2, DR3, DR4
U. Bastian (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
On the one hand, double stars are a nuisance for data reductions. On the other hand, they are scientifically interesting and important. After all, binaries probably constitute the majority of the overall stellar population. Thus they are crucial e.g. for our understanding of the formation of stars and planets in general. The Gaia mission sees, discovers, measures and parameterizes double stars - both optical pairs and physical binaries - in a surprising multitude of ways. Each of these ways poses an operational challenge as well as a scientific chance. Once fully exploited they will give a strongly revised picture of stellar binarity statistics. And they will remove all the disturbances caused by duplicity in the astrometric and photometric data of Gaia DR2. Gaia DR1 achieved an effective angular resolution (i.e.pair separations) of 2 arcsec, DR2 of 0.4 arcsec. But the actual optical resolution of the Gaia instrument is about 0.15 arcsec, and there are ways to detect and measure pairs down to the milli-arcsec level. All this can be done for hundreds of millions of stars, but it means a few more years of hard work by the Gaia data reduction consortium.

2019-07-04
11:15
The Star-Formation Properties of the Observed and Simulated AGN Universe: BAT vs EAGLE
Thomas Jackson (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Leading models of galaxy formation require AGN feedback to reproduce the properties of galaxies in the local Universe (i.e., colours; metallicities; galaxy stellar mass functions). Using extensive observations in the optical and Infra-Red of an X-ray selected sample of AGN taken from the SWIFT-BAT all-sky sample (z < 0.05), we have undertaken a multi-faceted comparison of the properties of local AGN to those predicted by the suite of EAGLE hydrodynamical simulations. We find that EAGLE can reproduce key aspects of the BAT AGN, including the distributions of their host masses, colours, star-formation rates and nuclear luminosities, as well as provide unique insight into the cosmic evolution of this population. Our studies help us identify the physics that make AGN special in both our local Universe and simulated ones.

2019-06-27
11:15
A new stellar feedback-driven model for synthetic nebular emission of clouds and galaxies
Daniel Rahner (ITA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Star formation is an inefficient process and in general only a small fraction of the gas in a giant molecular cloud (GMC) is turned into stars. This is partly due to the negative effect of stellar feedback from young massive star clusters. Recently, we introduced a novel 1D numerical treatment of the effects of stellar feedback from young massive clusters on their natal clouds, which we named WARPFIELD. With this model we can show that the minimum star formation efficiency, i.e. the star formation efficiency above which the cloud is destroyed by feedback, is mainly set by the average cloud surface density and that a star formation efficiency of 1-6 per cent is generally sufficient to destroy a GMC. These results imply that feedback alone is sufficient to explain the low observed star formation efficiencies of GMCs. We have now coupled our feedback model to the plasma code Cloudy and the radiative transfer code Polaris, which enables us to predict more than 100 emission lines from young star-forming regions and to compare these synthetic observations to observational surveys. In this talk I will address how individual objects (out of our database of thousands of simulated star-forming regions) evolve in various diagnostic plots (e.g. standard nebular emission line diagrams, so-called BPT-diagrams). Furthermore, I will present the evolution of whole galaxies as seen in synthetic observations.

2019-06-13
11:15
Star formation rate tracer lifetimes from synthetic observations of disc galaxy simulations
Daniel Haydon (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
A true understanding of star formation must come from the combined study of galaxy simulations and observational data: the physics built into our simulations are driven by observations. It is therefore critical to fully understand the emission tracers we rely upon. Synthetic observations are the key: they teach us how different emission tracers change with environment and, through comparisons with observational data, how well our simulations describe the real Universe. Through coupling the stellar population synthesis code SLUG to galaxy simulations, we can generate synthetic star formation rate tracer maps. The maps we generate have different metallicities and star formation rate surface densities, so we can explore and constrain the environmental effects on the timescale for which emission from different star formation rate tracers is visible. Real observations are often affected by extinction; it is therefore important to understand the extent that extinction could change these observed star formation rate tracer lifetimes. Using these timescales in observational application of “the uncertainty principle for star formation”, a new statistical method used in constraining the tracer lifetimes, it is possible to constrain the durations of different evolutionary phases of the star formation process and so better understand the physics of star formation and feedback on the cloud scale.

2019-06-06
11:15
ROME/REA: A gravitational microlensing search program for exo-planets beyond the snow-line on a global network of robotic telescopes
Yiannis Tsapras (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Planet population synthesis models predict an abundance of planets with semi-major axes between 1-10 au. At such distances, they lie at the edge of the detection limits of most planet finding techniques. Discovering these planets and studying their distribution is critical for understanding the physical processes that drive planet formation. ROME/REA is a gravitational microlensing project whose main science driver is to discover exoplanets in the cold outer regions of planetary systems. To achieve this, it uses a novel approach combining a multi-band photometric monitoring survey with reactive follow-up observations, exploiting the unique capabilities of the Las Cumbres Observatory (LCO) global network of robotic telescopes combined with a Target and Observation Manager (TOM) system. I will present the main science objectives and a technical overview of the project, including initial results.

2019-05-23
11:15
Into the darkness: charting the Zona Galactica Incognita with Cepheids
Istvan Dekany (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
One of the most concealed parts of the known Universe is the far side of our own Galaxy. Occupying only a few percent of the sky and disguised by interstellar dust, stellar tracers in these distant regions have been elusive until now. The exploration of these vast uncharted areas of the Milky Way by deep photometric surveys has just begun. We conducted a census of distant classical and type II Cepheids along the southern Galactic mid-plane using near-infrared photometry from the VVV (VISTA Variables in the Vía Láctea) Survey. After the complete revision of the survey's calibration, we leveraged the photometry to its full potential. A machine-learned classification of the Cepheids based on infrared light curves was performed for the first time, using a convolutional neural network. We have discovered 640 classical and over 500 type II Cepheids in the IV-th Galactic quadrant and behind the bulge, with up to 40 magnitudes of visual extinction. We also employ neural networks to estimate the Cepheids' reddening from sparse photometric data, revealing a space-varying extinction curve toward low Galactic latitude regions. The type II Cepheids were used to probe the 3-dimensional structure of the ancient stellar population in the inner bulge. Likewise, we used the classical Cepheids to trace large-scale spatial structures of the Galactic disk.

2019-05-16
11:15
The Physics Driving the Molecular Cloud Lifecycle during Galaxy Formation and Evolution
Melanie Chevance (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
The cloud-scale physics of star formation and feedback represent the main uncertainties in galaxy formation and evolution studies. We have recently shown that the multi-scale nature of the ‘’star formation relation’’ between the gas mass (surface density) and the star formation rate (surface density) allows us to directly measure fundamental quantities describing star formation and feedback on the cloud-scale, such as molecular cloud lifetimes, star formation efficiencies, feedback timescales, and coherence length scales. I will present the first systematic characterisation of the evolutionary timeline of molecular clouds and star-forming regions using the above method, across a wide variety of galactic environments. I will show that star formation is regulated by efficient stellar feedback, driving Giant Molecular Cloud (GMC) dispersal on short timescales (1-5 Myr) due to radiation and stellar winds, prior to supernova explosions. This feedback limits GMC lifetimes to about one dynamical timescale (10-30 Myr), with integrated star formation efficiencies of only a few percent. Our findings reveal that galaxies consist of building blocks undergoing vigorous, feedback-driven lifecycles, that vary with the galactic environment and collectively define how galaxies form stars. These observations settle a long-standing question on the multi-scale lifecycle of gas and stars in galaxies, and open up the exciting prospect of characterising cloud-scale star formation and feedback during galaxy formation and evolution.

2019-05-09
11:15
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Dimitris Gouliermis (ITA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
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2019-05-02
11:15
Hunt for Additional Planetary Companions: Revisiting Single Planet Systems with High Precision Doppler Spectroscopy
Trifon Trifonov (MPIA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
I will present results from a Doppler survey carried out with HARPS, FEROS and CARMENES for a sample of 45 stars, which were previously reported to have a single planetary companion in orbit. Most of these planetary discoveries were announced based on sparse radial velocity (RV) data samples, insufficient precision and sometimes incomplete phase coverage. Our survey aims to increase the observational efforts for these targets in an attempt to detect additional planets. So far, almost half of our targets are already showing strong indications of additional companions. For these systems with additional companions, we have conducted detailed statistical and dynamical analyses of the RV data and studied the long-term stability. We have a strong reason to believe that a significant fraction of the "single" RV planets are in fact either pairs of low-eccentricity near-resonant planets misinterpreted as a single planet or systems with a long-period massive companion, which can be revealed only by increasing the long temporal baseline of the observations. Since multiple planet systems (and their dynamical architecture) are important for probing planetary formation and evolution scenarios, I will emphasize the importance of further follow-up of RV planet hosts.

2019-04-25
11:15
Dorit Glawion (LSW)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Radio galaxies have emerged as a new very high energy (VHE) gamma-ray emitting source class on the extragalactic sky and started to deepen our understanding of the physical processes and the nature of active galactic nuclei (AGN) in general. Up to now there are six radio galaxies detected at gamma-ray energies above 100 GeV. With their jets not directly pointing towards us (i.e. ”misaligned blazars”), they offer a unique tool to probe into the nature of the fundamental (and often ”hidden”) radiation processes in AGN. The detection of fast flux variability in the TeV energy regime, for example, offers important information for our understanding of particle acceleration in AGN. In this talk I will discuss some of the observational results in the TeV range and the theoretical implications from recent years, including, e.g., the extreme short-term variability as seen in IC 310 and NGC 1275.

2019-04-18
11:15
Albireo, a pretty pair: Stable stellar marriage or celestial one-night stand?
Uli Bastian (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Gaia DR2 has added new aspects to a century-old question: Is Albireo (beta Cygni) a physical or an optical double? The road to a firm resolution of this question is amusingly littered with a number of misleading clues, reminiscent of a good crime movie. But recently I bet two pizzas with a dear colleague that the question will definitely be answered within the next 2-3 years. My bet is that it will turn out to be a physical double. This, by the way, is the sentimental hope of thousands and thousands of lovers of this object which is often quoted as the "most beautiful double star in the whole 4 pi sky."

2019-02-07
11:15
Lensing of '69 -- free gravitational lensing from its models
Jenny Wagner (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Strong gravitational lenses can map an extended background source to several highly distorted and magnified images. Analysing the properties of those images yields important information about the distribution of the deflecting mass and the background source. Common approaches to reconstruct the source or the deflecting mass distribution model the global properties of the source and the lens. They obtain a consistent description of the entire configuration by refining the model until it matches the observation to a predefined precision. We develop a new approach to infer local properties of the gravitational lens and to reconstruct the source only using the properties of the multiple images without assuming a lens or a source model. In the talk, I will introduce the method and its applications in comparison to standard lens modelling methods. Since our leading principle to separate data-based information from model assumptions can also be applied to a broader range of research questions, I will conclude with an outlook how this ansatz can be transferred to other topics, based on my former experience searching for open star clusters in the HSOY catalogue.

2019-01-31
11:15
Dwarf Galaxies in Cosmological Simulations: Fundamental Relations & the Effects of Environment
Christoph Engler (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Galaxies are shaped by their surrounding environment. Whether they reside in high-mass galaxy clusters, smaller groups or in the field, the characteristics of their baryonic components and the properties of their dark matter halos can appear very differently. These effects are especially distinct for dwarf galaxies, which are highly susceptible to external influence due to their low mass. We use the cosmological hydrodynamical simulations Illustris and IllustrisTNG to study the evolution of galaxies in a cosmological context and examine the connection of galaxy properties and their fundamental scaling relations, e.g. the mass-metallicity or the stellar-to-halo mass relation. We compare distributions of satellite galaxies in host halos of different mass with central galaxies and find the relations of these two galaxy populations to show clear offsets. But what properties of their environment are responsible for these differences? Why do properties of satellite galaxies show correlations with an increased scatter? And do these trends stay the same for different environments? Using quantities such as host mass, clustercentric distance or infall time to assess the strength of external effects, I will portray the impact of environment on galaxies as well as their surrounding dark matter halos.

2019-01-24
11:15
The universality of the physics governing massive cluster formation across cosmic time
Diderik Kruijssen (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Globular clusters are the relics of extreme star formation in high-redshift galaxies. Their enormous potential as tracers of high-redshift galaxy formation is broadly recognised, but concrete applications of this link have remained out of reach. The key missing ingredient has been to construct an end-to-end model for star cluster formation and evolution in a cosmological context. I will review recent efforts towards formulating models for globular cluster formation and evolution during galaxy formation, showcasing the variety of techniques used, as well as their strengths and weaknesses. I will then present results from the E-MOSAICS project, in which we carry out fully self-consistent, cosmological zoom-in hydrodynamics simulations of the co-formation and evolution of globular clusters and their host galaxies. This work has led to two crucial insights. The first is that the formation of young massive clusters and old globular clusters can be described by a single modelling framework, showing that globular clusters are the relics of regular star formation in high-redshift environments. The second is that the high-pressure formation environment of globular clusters has shaped a wide range of their present-day properties, enabling their direct use as tracers of high-redshift galaxy growth. We demonstrate how globular cluster metallicities, masses, ages, kinematics, and spatial distributions provide a new and exciting window for reconstructing the host galaxy merger history, distinguishing between in-situ and ex-situ galaxy growth, and probing the conditions of cloud-scale star formation and feedback at high redshift. Specifically, I will demonstrate the power of unifying cluster formation and destruction processes across cosmic time by using the E-MOSAICS simulations to derive the formation and assembly history of the Milky Way, culminating in the reconstruction of its merger tree.

2019-01-17
11:15
A numerical study on the distribution of the satellite debris in the MW environment
Matteo Mazzarini (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
The interaction of the Milky Way (MW) satellite galaxies with the Galactic environment produces tidal tails that are observed as denser concentrations in the sky (e.g. like in the case of the Magellanic Clouds tails or the Sagittarius dwarf stream). In this work we address via numerical simulations the distribution of dark matter (DM) and baryonic tidal streams in the Milky Way environment. We model our initial conditions (ICs) according to the Aquarius A2-to-F2 simulations and previous literature, employing satellite models made both of baryons and DM. We find that most of the baryons are still inside their progenitors, while most of DM is easily stripped and redistributed in the MW environment. We also find that the stripped baryonic debris ends up mostly in the inner regions of the MW halo and that it is flatter than the DM debris. A look at the angular distribution of the debris shows that it is oriented at specific angles with respect to the Galactic disk. This is found also rotating the satellites of 90 degrees with respect to their previous positions and running the simulations again. This result surprisingly contrasts with the expectations of a randomly distributed debris. Next, we move to extract ICs from latest cosmological simulations and to switch on gas dynamics and feedback to study the impact of baryonic physics on the distribution of satellite debris in the MW environment.

2019-01-10
11:15
CEMP stars - a new way of obtaining metallicities and classifications
Camilla Hansen (MPIA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Carbon enhanced metal-poor (CEMP) stars are low-mass stars formed throughout the history of the Universe. They are found in various environments from dwarf galaxies, to more massive galaxies like the Milky Way. These carbon rich stars can occur as single, very old, possibly bona fide second generation stars, or in binary systems, but they also occur as variable RR Lyr stars. This class of stars is therefore important to understand, as one can trace the chemical environments at different times from the first low-mass stars that formed shortly after Big Bang up to today. The CEMP stars are subcategorised depending on their heavy element (Z>30) content, and they tend to show an enhancement in either slow or rapid neutron-capture elements (or both) or a complete lack of heavy elements. These CEMP stars are typically cool stars with strong molecular bands obliviating the usual chemical imprints from the smaller atomic absorption features. It is therefore challenging to analyse this class of stars. Here I will present a new way of deriving metallicities for CEMP stars as well as a new fast way of subclassifying the CEMP stars into their various subgroups, which in turn allows for a first assessment of their binary nature using single exposure low-to-medium resolution spectra (i.e., follow-up observations tracing radial velocity variations may not be needed). These new methods may prove highly relevant in the era of large surveys where a fast chemical analysis is sought after. Finally, I will use the heavy elements to trace the extinct sources that enriched the long-living CEMP stars.

2018-12-13
11:15
Phylogeny of stars and the evolution of the Milky Way
Paula Jofre (Chile)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Chemical elements are produced by stars, making stellar generations always more metal-rich with time. Hence, stellar generations pass information from one to the other one via their chemical compositions. This information provides a fundament for the motivation to create a spectroscopic survey of stars in the Milky Way. The chemical composition of thousands of stars, combined with their dynamical properties and their ages, serve to constrain the models of formation and evolution of our Galaxy. In this talk, I will first review the state-of-the art of the precision and accuracy of such industrial stellar abundance measurements. Then I will present a novel method to visualise and interpret these datasets, which is based on phylogenetic trees, and has been so far tested in a dataset of higher precision and accuracy.

2018-12-06
11:15
The Dwarf Galaxy Version of the Twin Paradox
Thorsten Lisker (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Two dwarf galaxies begin their lives in dark matter haloes of identical mass. They start their evolution with identical baryonic properties - but then one twin falls into a galaxy cluster, while the other one stays out in the field. The cluster environment alters the baryonic characteristics of the first dwarf: ram pressure removes the gas, harassment increases the random motion of the stars. At some later epoch, the second dwarf also enters the same cluster, and they happen to meet at a crossing point of their orbits. When comparing them, the observers notice that the first dwarf looks much older than the second one, concluding that the strong gravitational force inside the cluster was responsible for that. - But wait: is it even true that a dwarf in an overdense region begins with the same initial properties than a dwarf in an underdense region? Aren't there more evolutionary effects than just the influence of the galaxy cluster? In this talk, I will touch on several aspects of our understanding - as well as some misunderstandings - of dwarf galaxy evolution under environmental influence. Paradoxically, it seems that at least a fraction of the old dwarfs arrived only recently to the cluster environment.

2018-11-29
11:15
Adding the Infall-time Dimension to our Understanding of Environmental Effects in Groups and Clusters using Phase-space
Rory Smith (KASI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
A phase-space diagram of a cluster can be a simple plot of clustocentric velocity versus clustocentric radius for each member of a cluster. Using a suite of cosmological hydrodynamical simulations of clusters, we investigate where simulated galaxies fall in phase-space. We find that, by comparing populations of galaxies in different regions of phase-space, we can systematically change the mean infall time of that population into their host. This is very useful for understanding environmental effects as the presence of recent infallers mixed in with ancient infallers tends to camouflage the true environmental impact, and we are able to clearly reveal it using phase-space. We then apply this tool to observations of galaxies in groups and clusters to study gas stripping, jelly-fish galaxies and tidal-mass loss. We also consider a large sample of SDSS satellite galaxies and study how the time they spend in their host impacts on luminosity-weighted ages, specific star formation rates, metallicity and alpha-abundances of the satellites. Using phase-space, we reveal clear signatures that even low-mass hosts can impact the stellar mass growth of their massive satellites, and see tentative hints of the effect of protoclusters and their surroundings on initial galaxy growth as early as redshift three. In short, we demonstrate how phase-space diagrams are essential tools for understanding environmental effects acting on satellite galaxies.

2018-11-27
11:15
Stellar halos and low surface brightness features in the galaxy outskirts from deep VST surveys
Enrichetta Iodice (INAF-Osservatorio Astronomico di Capodimonte)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
In the recent years, a big effort was made to develop deep photometric surveys aimed at studying galaxy structures down to the faintest levels of surface brightness. The VST Early-type GAlaxy Survey (VEGAS) has played a pivotal role in the field, by providing new insight on the faint regions of galaxies and on dwarf galaxies in all environments. About 30% of the VEGAS observing time was dedicated to the Fornax deep survey (FDS), the new multi-band deep survey of the Fornax cluster, covering the whole cluster out to the virial radius, with an area of 26 square degrees around the central galaxy NGC1399 and including the SW subgroup centred on the other bright member NGC 1316. With FDS we can map galaxy stellar halos down to μg ≃ 29−31 mag/arcsec2, detect new and faint (μg ≃ 28−30 mag/arcsec2) features in the intracluster space and trace the spatial distribution of candidate globular clusters inside ∼ 0.5 deg2 of the cluster core. Recently, the light and colour distribution of all the bright early-type galaxies inside the virial radius of FORNAX have been studied and a first comprehensive view of the galaxy structure and evolution as function of the cluster environment has been provided. In this talk I will review the main results obtained from VEGAS/FDS surveys.

2018-11-22
11:15
How to measure Stellar Masses with Astrometric Microlensing: Predictions based on Gaia DR2
Jonas Klueter (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Astrometric gravitational microlensing is an excellent tool to measure the mass of a star. Even though astrometric microlensing events with measurable deflections are extremely rare, there is one big advantage compared to photometric microlensing events: Astrometric microlensing events can be predicted, based on highly accurate positions, proper motions and parallaxes of lens star and source star. Through a time series of very precise measurements of the lensed background star’s position during the passage of the foreground lensing star it is possible to determine the mass of the lensing star with an accuracy of a few percent. In this talk I will present the analysis of the recent published Gaia data release 2 (DR2) and the prediction of a number of astrometric microlensing events in the near future. In particular I will present an ongoing microlensing event caused by the high proper motion star Luyten 143-23.

2018-11-15
11:15
Ultra-Diffuse Galaxies - A New Galaxy Type?
Eva Grebel (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Ultra-diffuse galaxies (UDGs) are galaxies with very low surface brightnesses and large physical sizes. Originally discovered a few decades ago, they were mostly ignored until 2015 when the interest in these objects was suddenly re-ignited. Since then, numerous dedicated studies have appeared each year, and meanwhile UDGs are being routinely detected in galaxy surveys. Nonetheless, the origin and nature of UDGs remain unresolved. Are they "failed" Milky-Way-type galaxies quenched by cluster environments? Are they highly dark-matter-dominated dwarfs? Is their evolution regulated by internal or by external effects? Are they indeed a new class of galaxies or just the tail of the low-surface-brightness galaxy population? The intensified research during just the last four years has revealed a number of intriguing and varied properties, which I will attempt to summarize in my talk along with possible formation pathways.

2018-11-08
11:15
The Milky Way disk in the framework of a semi-analytic model: Gaia data, forward modelling approach, and perspectives
Kseniia Sysoliatina (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
We use highly precise astrometric parameters and photometry provided in Gaia DR1 and DR2, and also high-resolution spectroscopy from surveys such as RAVE and APOGEE, in order to derive new constraints on the Milky Way disk evolution as viewed in the frame- work of a semi-analytic chemo-dynamic model (JJ model) calibrated in the solar neighborhood. First, we discuss the possibility to extend the JJ model to Galactocentric distances beyond solar, R=[4,12] kpc, and compare the predicted trends in stellar distribution with the corresponding trends identified through observations or predicted by N-body simulations. Second, we test the model in the solar cylinder against the thin-disk sample selected from the Gaia DR1 and RAVE data. Based on the JJ model, we simulate this sample via the forward modelling technique. In particular, we convert the predicted vertical density laws of the thin-disk populations into a mock sample making use of a decreasing SFR and a three-slope broken power law IMF. The obtained mock populations are reddened with a 3D dust map and subjected to the selection criteria corresponding to the RAVE and Gaia DR1 observational limitations as well as to additional cuts applied to the data sample. We calculate the quantities of interest (star counts, Hess diagrams, velocity distribution functions) separately at different heights above the Galactic plane also accounting for the observational distance errors. We also check and confirm the consistency of our results with the newly available Gaia DR2. In the end, the future work in this direction will be discussed.

2018-10-25
11:15
Mothership's calling back: dwarf galaxies, dark matter, and a bit more.
Veronica Lora (Mexico)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Dwarf spheroidal galaxies are dark matter dominated. Therefore, they are ideal to test any dark matter model. In this colloquium I will talk about my previous work on stellar substructures in dwarf spheroidal galaxies and how important they are to constrain dark matter models. I will continue to present our newly discovered stellar substructures in Sextans, Carina, Leo I and Leo II. Finally, I will present my current and future work.

2018-10-25
11:15
Mothership's calling back: dwarf galaxies, dark matter, and a bit more.
Veronica Lora (Mexico)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Dwarf spheroidal galaxies are dark matter dominated. Therefore, they are ideal to test any dark matter model. In this colloquium I will talk about my previous work on stellar substructures in dwarf spheroidal galaxies and how important they are to constrain dark matter models. I will also present our newly discovered stellar substructures in Sextans, Carina, Leo I, and Leo II. Finally, I will talk about my current and future work.

2018-10-18
11:15
Turbulent lives of massive stars
Fabian Schneider (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Stars are the building blocks of the Universe and have transformed the pristine Universe into the one we live in today. Massive stars in particular play a central role because they are cosmic powerhouses that give rise to copious amounts of ionising radiation, chemical elements and kinetic energy. They die as the most powerful explosions in the Universe and – if they lived in binary systems – their compact remnants are at the heart of gravitational wave astronomy. To understand the role of massive stars in the cosmos, a comprehensive  picture of their lives and final fates is essential, but is as yet  seriously incomplete. I will highlight some of the most pressing challenges in massive star research and then focus on two of them. First, I will discuss the stellar initial mass function of stars in the mass range 15-200 solar masses as observed in the local 30 Doradus starburst and implications for the maximum birth mass of stars and stellar feedback. Second, I will explain how stellar mergers may explain some of the enigmatic OBA stars that have strong, large-scale magnetic fields.

2018-10-11
11:15
Accretion and Feedback in the Formation of Massive Stars
Rolf Kuiper (Institut für Astronomie und Astrophysik, Universität Tübingen)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
In the course of their accretion phase, massive (proto)stars impact their natal environment in a variety of feedback effects such as thermal heating, protostellar jets and outflows, radiation forces, as well as photoionization and HII regions. Here, I present our most recent simulation results in terms of the relative strength of the feedback components and the size of the reservoir from which the forming stars gain their masses. For the first time, these simulations include all of the feedback effects mentioned above which allows us to shed light on the physical reason for the upper mass limit of present-day stars. Furthermore, we predict the fragmentation of massive circumstellar accretion disks as a viable road to the formation of spectroscopic massive binaries and the recently observed strong accretion bursts in high-mass star forming regions.

2018-09-13
11:15
A Song of Fire and Feedback: The Importance of AGN-driven Outflows in Quenching Star Formation in Massive Galaxies
Michael Tremmel (Yale University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Using multiple sets of “genetically modified” galaxy simulations, as well as the Romulus large-scale cosmological simulations, I demonstrate the importance of AGN-driven outflows, which often reach out to 10s kpc and beyond, in regulating and quenching star formation in massive galaxies. Galaxies that quench at high redshift can have very complicated star formation and supermassive black hole growth histories, as gas cooling and accretion is in constant struggle against AGN feedback. Using RomulusC, the highest resolution simulation ever performed of a brightest cluster galaxy, I examine the physics behind how large-scale, powerful outflows are able to suppress star formation in one of the most extreme environments: the center of a cool core cluster. Finally, I will discuss the role that AGN-driven outflows play in shaping the circum-galactic medium (CGM) of Milky Way-mass galaxies and the extent to which we can look to the CGM for signatures of AGN feedback.

2018-07-26
11:15
The life of star clusters, from birth to dissolution: a new approach
Bekdaulet Shukirgaliyev (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
We study the long-term evolution of star clusters in the Galactic disk, starting from their birth in gaseous clumps until their complete dissolution in the tidal field. To do so, we combine the semi-analytic ''local-density-driven'' cluster formation model of Parmentier & Pfalzner (2013) with direct N-body simulations of clusters following instantaneous gas expulsion. We look at the relation between cluster dissolution time and cluster initial mass. The model clusters formed with a high star formation efficiency (SFE -- i.e. gas mass fraction converted into stars) follow a tight mass-dependent relation, in agreement with previous theoretical studies. However, the low-SFE models present a large scatter in both the initial mass and the dissolution time, and present a shallower mass-dependent relation than high-SFE clusters. Combining both populations of clusters, i.e. high- and low-SFE ones, with domination of the latter, yields a cluster dissolution time for the solar neighborhood in agreement with that inferred from observations, without any additional destructive processes such as giant molecular cloud encounters. An apparent mass-independent dissolution relation may emerge for our low-SFE clusters when we neglect low-mass clusters (as expected for extra-galactic observations), although more simulations are needed to investigate this aspect.

2018-07-19
11:15
From point clouds to physics: Dynamical state and gravitational lensing properties of Illustris galaxy clusters
Aksel Alpay (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
We investigate the six most massive Illustris galaxy clusters in the Illustris simulation (Nelson et al. 2015) at z=0.2 and z=0.44 in terms of their dynamical state and their strong gravitational lensing properties. For this, we present illcrawl, a new software for the efficient reconstruction and visualization of Illustris data with GPUs. illcrawl is used to apply to our clusters the method by Mantz et al. (2014), which estimates the dynamical state of galaxy clusters based on their X-ray morphology. We find that this method considers those clusters as relaxed that appear undisturbed in terms of their X-ray gas, indicating that this method works as expected with simulated galaxy clusters. The clusters that are considered un- relaxed feature prominent “bubbles” in the X-ray gas due to the Illustris AGN feedback model. The virial ratios for all clusters hint however at mostly relaxed systems. We also show how regions can be determined where strong gravitational lensing effects are expected. We find that strong lensing in Illustris clusters is expected mostly on scales of individual galaxies. All our detected lensing regions are observable, i.e. we find no “dark lenses”. All our strong lensing regions show a tight correlation between stellar content and total mass.

2018-07-12
11:15
The stellar fossil record of dE galaxies
Francesco La Barbera (Osservatorio Astronomico di Capodimonte)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Dwarf elliptical galaxies (dEs) represent a fascinating class of systems, outnumbering other classes of galaxies in groups and clusters. Nevertheless, due to the faint surface brightness level of dEs, no much is known about their stellar population properties. I will present a stellar population study --- the most detailed one performed so far -- for the typical dE galaxy NGC1396, in the Fornax cluster. Thanks to the exquisite quality of VLT-MUSE IFU spectroscopy, we have studied a variety of optical and NIR absorption features at different galactocentric distances in NGC1396, to constrain (for the very first time) the star-formation history, metallicity content, chemical abundances, and stellar IMF, in this system. I will discuss how these results provide a unique imprint of the formation and evolution processes of dEs.

2018-07-05
11:15
A systematic search for tidal features around nearby galaxies: Enhanced SDSS imaging of the Local Volume.
Gustavo Morales (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
In hierarchical models of galaxy formation, stellar tidal streams are expected around most galaxies. Although these features may provide useful diagnostics of the LCDM model, their observational properties remain poorly constrained. Statistical analysis of the counts and properties of such features is of interest for a direct comparison against results from numerical simulations. We aimed to study systematically the frequency of occurrence and other observational properties of tidal features around nearby galaxies. The approach featured here is based on a visual classification of diffuse features around a sample of nearby galaxies, using a post-processing of optical survey imaging optimized for the detection of low-surfacebrightness stellar structure. At the limiting surface brightness of this sample, 14 to 17% of the galaxies exhibit evidence of diffuse features likely to have arisen from minor merging events. For simulated images, the frequency is roughly 16 to 19%. Our technique recovers all previously known streams in the selected sample and yields a number of new candidates. We conclude that this methodology provides a reliable foundation for the statistical analysis of diffuse circumgalactic features in wide-area imaging surveys, and for the identification of targets for follow-up studies.

2018-06-22
12:00
The Stellar Halos of Nearby Early-type Galaxies: Highlights from the SLUGGS survey
Prof. Duncan Forbes (Swinburne University, Melbourne)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Using images from Subaru and spectroscopy from Keck, the SLUGGS survey aims to understand the formation and evolution of massive early-type galaxies. For a sample of 25 galaxies, we probe the detailed kinematics and metallicities of their field stars to 3-4 effective radii and their globular clusters to 8-15 effective radii. From these data we have derived 2D maps of the halo stellar kinematics and metallicity. We find changing kinematic signatures as we probe from the galaxy inner to halo regions. Using JAM models we have derived the mass density slope and compare it to the latest cosmological simulations. The SLUGGS survey has also collected over 4000 globular cluster radial velocities (the largest sample to date). From this data we have derived the dynamical mass and dark matter fractions for our galaxies. We compare our results with the latest predictions from the Illustris simulations. We also explore the kinematics and scaling relations of globular cluster systems. These results are placed in the context of two-phase galaxy formation. And if time allows, I will briefly mention recent work on ultra-diffuse galaxies using the Keck telescope.

2018-06-21
11:15
Short-lived radioisotopes in meteorites from Galactic-scale correlated star formation
Dr. Yusuke Fujimoto (Australian National University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Meteoritic evidence shows that the Solar system at birth contained significant quantities of short-lived radioisotopes (SLRs) such as 60Fe (with a half-life of 2.6 Myr) and 26Al (with a half-life of 0.7 Myr) produced in supernova explosions and in the Wolf-Rayet winds that precede them. Proposed explanations for the high SLR abundance include formation of the Sun in a supernova-triggered collapse or in a giant molecular cloud (GMC) that was massive enough to survive multiple supernovae (SNe) and confine their ejecta. However, the former scenario is possible only if the Sun is a rare outlier among massive stars, while the latter appears to be inconsistent with the observation that 26Al is distributed with a scale height significantly larger than GMCs. In this talk, we present a high-resolution chemo-hydrodynamical simulation of the entire Milky Way Galaxy, including stochastic star formation, HII regions, SNe, and element injection, that allows us to measure for the distribution of 60Fe/56Fe and 26Al/27Al ratios over all stars in the Galaxy. We show that the Solar System's abundance ratios are well within the normal range, but that SLRs originate neither from triggering nor from confinement in long-lived clouds as previously conjectured. Instead, we find that SLRs are abundant in newborn stars because star formation is correlated on galactic scales, so that ejecta preferentially enrich atomic gas that will subsequently be accreted onto existing GMCs or will form new ones. Thus new generations of stars preferentially form in patches of the Galaxy contaminated by previous generations of stellar winds and supernovae.

2018-06-14
11:15
A year at ARI chasing BHATS (black holes at all the scales)
Manuel Arca Sedda (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Globular and nuclear clusters are ideal nurseries for black holes (BHs) and other compact stellar remnants and are sufficiently crowded to favor the formation of tightly bound pairs, which in some cases can merge and release observable gravitational waves. Globular clusters are thought to be a perfect birthplace for intermediate mass BHs (IMBH, mass range 10^2-10^5Msun), while some nuclear clusters offer a unique view on the interplay between stars and supermassive BHs (SMBH, mass range 10^5-10^9 Msun). Unfortunately, defining a robust way to assess the presence of these compact objects in dense clusters on an observational basis is a hard task to accomplish. In this talk, I will discuss the impact of stellar BHs and IMBHs evolution on globular and nuclear clusters observational properties. For globulars, it is possible to define a fundamental plane'' that connects their observational quantities to the main properties of their BH populations. This fundamental plane suggests that ~30 Galactic globular clusters may be currently harboring a noticeable number of BHs. Regarding nuclear clusters, I will discuss how the processes that govern their formation can be connected to observations using our Galactic Centre as a reference case.

2018-06-07
11:15
Mapping the ionized ISM in nearby galaxies
Kathryn Kreckel (MPIA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
The ionized interstellar medium (ISM) provides crucial insights into understanding baryon cycling within disk galaxies and tracing radiative and mechanical feedback from young massive stars. With new VLT/MUSE optical integral field spectroscopy, the PHANGS team now has a wealth of emission line maps that trace different ionization sources and physical conditions across nearby disk galaxies at the 50pc spatial scales needed to resolve individual HII regions. I will show my results from NGC 628, comparing the arm and interarm star formation, classifying the PN and SNR populations, and tracing variations in the star formation efficiency.

2018-05-24
11:15
OGLE-ing the Magellanic System: Three-Dimensional Structure Using Classical Pulsators
Anna Jacyszyn-Dobrzeniecka (Warsaw University Observatory, Poland)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
I will present the three-dimensional structure of the Magellanic System using over 9000 classical Cepheids (CCs) and almost 23000 RR Lyrae (RRL) stars from the OGLE Collection of Variable Stars. The vast coverage of the OGLE-IV data and the high completeness of the sample allowed us to study the Magellanic System in great detail. The CC distribution shows that, contrary to previous results, the LMC bar is not offset from the galaxy's plane. Moreover, the northern arm is located closer to us than the overall sample. The CCs in the SMC have a non-planar distribution that can be described as an ellipsoid extended almost along the line of sight. RRL stars reveal a very regular distribution in both Magellanic Clouds. In the Magellanic Bridge, there is no evidence of an actual physical connection between the Clouds in the RRL distribution. We only see the two halos overlapping. The few CCs in the Bridge seem to form a genuine connection between the Clouds. Their on-sky locations are well correlated with young stars and the neutral hydrogen distribution.

2018-05-17
11:15
Exercises in Galactic Dynamics
Walter Dehnen (Leicester University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Part 1: I present an analysis of Gaia DR1 data in terms of the vertical velocity of stars as function of their radial position and angular momentum. The mean vertical velocity shows a strong evidence for both a stellar warp and a vertical bending wave, which could have been triggered by interaction with SgrA*. I may also report on ongoing work using Gaia DR2. Part 2: I present simulations and analytical insight into the formation of tidal debris from a satellite on a disc orbit, such as 47Tuc. The debris from such a progenitor does not form a thin stream or tail, but a vertically extended ribbon wrapping around the Milky Way and with vertical extent comparable to that of the progenitor's orbit. I discuss the implications for detection and interpretation of such tidal ribbons.

2018-05-03
11:15
Gaia DR2: The first "real" Gaia catalogue
Michael Biermann (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
On April 25, just a few days ago, the second Gaia Data Release was published. I will present the contents and properties of this outstanding Gaia catalogue, and exhibit a glimpse of the scientific potential of Gaia DR2.

2018-04-26
11:15
Using Globular Cluster Stellar Populations to Understand Galaxy Formation
Christopher Usher (Liverpool John Moores University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Globular clusters are important tools to help us understand how galaxies form and evolve. Globular cluster formation tells us about the conditions of extreme star formation while their survival from high redshift tells us about the processes of galaxy assembly. Being much brighter than red giant stars, globular clusters allow the stellar populations of galaxies to be studied at much greater distances, and thus a wider range of galaxy masses, environments and morphologies to be studied than can be with resolved stars. Using data from the WAGGS survey of massive star clusters in the Milky Way and its satellite galaxies and the SLUGGS survey of the globular cluster systems of massive early-type galaxies, I will talk about how we can measure the metallicities of globular clusters using the strength of the calcium triplet in integrated light. Using globular cluster metallicity distributions and the relationships between globular cluster colour and metallicity, I will present evidence that different galaxies with similar masses experienced different formation histories. I will compare these observations with the predictions of the E-MOSAICS cosmological simulations of the formation of globular cluster systems. I will also talk about how the E-MOSAICS simulations have allowed to us to understand how globular cluster colour distributions vary

2018-04-19
11:15
Central stars of mid-infrared nebulae discovered with Spitzer and WISE
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Massive stars lose mass in the form of stellar winds as well as outflows and instantaneous ejections, caused by various processes in binary systems. The lost material forms circumstellar nebulae of diverse shapes, which could be detected with modern infrared telescopes. We present the results of search for massive stars through detection of compact mid-infrared nebulae with Spitzer and WISE. Follow-up spectroscopy of the central stars of these nebulae led to the discovery of several dozens of candidate luminous blue variables (LBVs). Four of them were confirmed to be bona fide LBVs by means of spectroscopic and photometric monitoring. The majority of other central stars of mid-infrared nebulae turns out to be either blue supergiants or Wolf-Rayet stars of the late nitrogen sequence.

2018-02-08
11:15
Search for Star Cluster Age Gradients Across Spiral Arms of Three LEGUS Disk Galaxies
Fayezeh Shabani (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Understanding how spiral arms form in disk galaxies is a long-standing issue in astrophysics. One of the main theories for explaining the formation of spiral patterns in disk galaxies is the stationary density wave theory. This theory predicts the existence of an age gradient across the arms. We use the stellar cluster catalogues of the galaxies NGC 1566, M51a, and NGC 628 from the Legacy Extragalactic UV Survey (LEGUS) program. In order to test the possible existence of an age sequence across the spiral arms, we quantified the azimuthal offset between star clusters at different ages in our target galaxies. We found that NGC 1566, a grand–design spiral galaxy with bisymmetric arms and a strong bar, shows a significant age gradient across the spiral arms that appears consistent with the prediction of the stationary density wave theory. In contrast, M51a with its two well–defined spiral arms and a weaker bar does not show an age gradient across the arms. We believe that the spiral structure of M51a is not the result of a stationary density wave with a fixed pattern speed. In this case, tidal interactions could be the dominant mechanism for the formation of spiral arms. We also found no offset in the azimuthal distribution of star clusters with different ages across the weak spiral arms of NGC 628.

2018-02-01
11:15
What can the Dwarf and Compact Stellar System Population of the Fornax Galaxy Cluster tell us about its Formation History?
Yasna Ordenes (PUC Chile, ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Given its proximity, the Fornax cluster is a goldmine for studying the formation and evolution of galaxies and compact stellar systems (CSSs), globular clusters (GCs), and ultra compact dwarf galaxies (UCDs) in a cluster environment. We have obtained deep multi-passband imaging as part of the Next Generation Fornax Survey in optical (u′g′i′) bands with DECam and near-infrared (JKs) bands with VIRCam. I will present the latest results of my PhD thesis work. First, I will start with the dwarf galaxy population in the outer Fornax cluster region where we have discovered 271 dwarf galaxies. Second, I will show the results for the nuclei in Fornax dwarf galaxies where we have detected a bimodal nuclei mass distribution and the nucleus mass vs. galaxy mass relation becomes shallower for less massive galaxies. Finally, I will show the current status of the CSSs research in the central region, the spatial distribution in terms of age, metallicity and mass distribution.

2018-01-25
11:15
Halo vs. Bulge: a mixed bag of chemical enrichments
Andreas Koch (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
The bulge is one of the oldest, yet very metal rich Galactic components suggesting that it experienced early, rapid chemical enrichment. Large spectroscopic surveys are beginning to unearth progressively more metal-poor stars ([Fe/H] < -2 dex) that are predicted to exist in models of galactic evolution, according to which the first stars did form in the bulge. In this talk, I present an overview of such endeavours to detect and characterize such stars. While the chemical abundances of most candidates vastly overlap with those of halo stars, our sample also contains carbon-rich (CEMP) stars -- the first detections of such objects in the bulge. Based on kinematic considerations, I further discuss whether similar claims of metal-poor star detections are true habitants of the bulge or rather halo stars passing through the central regions. As a second, fascinating class of objects, I will then focus on globular clusters and their multiple stellar populations, and their role in the build-up of the Milky Way's halo.

2018-01-18
11:15
Were the most compact and most diffuse stellar systems in galaxy clusters both formed by tidal stripping?
Carolin Wittmann (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
We use deep optical wide field imaging data of the nearby Perseus and Fornax galaxy clusters to analyse the most compact and most diffuse stellar systems for signs of tidal stripping. In the Fornax cluster core we examine a population of spectroscopically confirmed ultra-compact dwarf galaxies (UCDs) and probe whether they could be the remnant nuclei of destroyed nucleated early-type dwarfs. Although we do not detect diffuse tidal debris around any of the UCDs, a significant fraction of our sample exhibits distorted outer structures. In the core of Perseus we identify a population of low-surface brightness dwarf galaxy candidates of the kind named 'ultra-diffuse galaxies'. We investigate whether they show signs of current tidal disruption or whether they might be protected by a large dark matter content. Our sample seems surprisingly intact, with only very few candidates showing possible signs of current disruption. We discuss the importance of tidal stripping for the formation of both the compact and the diffuse cluster populations by comparison to the predicted timescales and frequency of disruption events from simulations.

2018-01-11
11:15
Studying the effects of stellar evolution on the chemical composition of stars in M67
Clio Bertelli Motta (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
The chemical composition of a star is not constant throughout its life. It changes due to different evolutionary processes such as atomic diffusion between the main sequence and the subgiant branch, the first dredge-up along the subgiant and lower red giant branch, and extra-mixing taking place after the luminosity bump on the red giant branch. All these effects are hard to measure observationally, since a fairly large sample of stars belonging to the same stellar population and in different evolutionary stages is required. Being built up of a single stellar population, open clusters are therefore perfect laboratories for the study of stellar evolution. We present a study of the effects of atomic diffusion on the stars of the well-known old open cluster M67 using high-resolution spectroscopic data from the Gaia-ESO Survey. We compare the observational results to predictions from theoretical models and discuss them with respect to broader implications. In fact, atomic diffusion sets the resolution achievable by chemical tagging methods, since stars cannot be traced back to their cluster of origin with a precision higher than the variation in surface chemical composition caused by evolutionary effects. Furthermore, we will present the results of a study on the chemical composition of blue straggler stars and evolved blue straggler stars in M67 using data from APOGEE DR14. We will discuss the implications of these results in the investigation of the formation scenario and the evolution of the objects under study.

2017-12-21
11:15
The Near-IR RR Lyrae Census of the Southern Galactic Plane
Gergely Hajdu (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
RR Lyrae stars are one of the best tracers of the oldest populations of stellar systems. The RR Lyrae content of the low-latitude regions of the Galactic disk is especially interesting in light of the formation of the thick disk. Unfortunately, severe extinction has prohibited the exploration of the variable star content of this region so far, but with the completion of the Vista Variables in the Via Lactea (VVV) survey, we can bring light to these questions. Machine-learning techniques have been adopted to classify RR Lyrae variables based on their light-curve shapes in the light curves of the approximately 200 million point sources in the VVV, resulting in the detection of 1892 high-confidence RR Lyrae stars. Utilizing newly developed techniques, we have measured mean magnitudes of these variables, enabling the determination of accurate distances for each of them. Furthermore, we show that the mean iron abundance of RR Lyrae variables can be determined from their K-band light-curve shapes, which we use to explore different sub-populations of the newly found variables. The structure of the metallicity distribution of this sample bears remarkable similarity to that of the ARGOS survey, covering the much younger red clump stars of the Galactic plane and bulge. The few differences can be easily understood as a lack of RR Lyrae stars with supersolar metallicities, as well as halo interlopers enhancing the RR Lyrae metallicity distribution at the lower metallicity end. The weight of each sub-population evolves with Galactocentric distance, supporting inside-out formation scenarios.

2017-12-14
11:15
The local rotation curve of the Milky Way based on SEGUE and RAVE data
Kseniia Sysoliatina (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
The measurement of the Galactic rotation curve provides a powerful tool for constraining the mass distribution in the Milky Way and enters various branches of Galactic kinematics as an essential ingredient. The Galactic rotation curves obtained by different methods are still not in a prefect agreement. In our work we investigate the shape of the rotation curve in the extended solar neighborhood and test it for presence of any peculiarities just outside the solar orbit as has been reported by some authors. We use RAVE data to determine the solar peculiar velocity and the radial scalelengths for the three populations of different metallicities representing the Galactic thin disc. For the same metallicity populations we construct the rotation curve in the range of Galactocentric distances 7-10 kpc for the sample of G-dwarfs from SEGUE. This work demonstrates the importance and non-triviality of the asymmetric drift correction while inferring the rotation velocity from the dynamically heated tracers. We find that the local rotation curve is essentially flat, thus the presence of any peculiarities in the rotation curve shape just outside the solar radius can be disproved.

2017-12-07
11:15
Small-scale cosmology with dwarf galaxies
Oliver Müller (Universität Basel)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Dwarf galaxies are tracers of the fine-structure of the large-scale structure of the universe, but their predicted abundance and distribution from simulations is in conflict with observations in the Local Group, today's best studied group of galaxies. Recently, two planes of satellites were discovered around Centaurus A, providing a unique opportunity to test cosmological predictions beyond the Local Group. We surveyed the complete Centaurus Group with the Dark Energy Camera, doubling the census of dwarf galaxies in the group. We found evidence that these satellite planes are co-rotating, posing a challenge to LambdaCDM. Comparison with high-resolution dark matter simulations shows that this finding is highly significant. Furthermore, I will connect this to the recently established and unexpected relation between the number of dwarf galaxies and the size of the bulge of the host galaxy in the context of LambdaCDM cosmology and MOND.

2017-11-30
11:15
The non-linearity and unintuitive nature of stellar feedback
Eric Pellegrini (ITA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
The kinematics and chemodynamics of molecular clouds post-star formation are driven by stellar feedback and interaction with the natal ISM. Historically, this evolution has been studied from overly simplistic arguments using energy and momentum conservation. The reality is more complicated, and nonlinear. I will discuss true nature of cloud evolution undergoing stellar feedback, including the time evolution of the cloud emission in classic emission line diagnostic diagrams. Through these diagrams we hope to develop methods to recover the history of star formation.

2017-11-23
11:15
Young, metal-enriched cores in early-type dwarf galaxies in the Virgo cluster
Linda Urich (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Early-type dwarf galaxies are not simply featureless, old objects, but were found to be much more diverse, hosting substructures and a variety of stellar population properties. To explore the stellar content of faint early-type galaxies, and to investigate in particular those with recent central star formation, we study colours and colour gradients within one effective radius in optical and near-infrared bands for 120 Virgo cluster early-type galaxies. Twelve galaxies turn out to have blue cores and we find them to have the strongest age gradients, i.e., the youngest central regions. Moreover, the metallicity gradients of almost all early-type dwarf galaxies are found to be negative, indicating metal-enriched central regions. A cluster environment is expected to influence such low-mass galaxies through tidal forces through interactions with other galaxies (harassment) as well as through the ram-pressure of its hot intracluster medium (ram-pressure stripping). Therefore, we are comparing the observed stellar population gradients with model predictions, in order to investigate how ram-pressure stripping and harassment can affect a galaxy's star formation activity and stellar population distribution depending on the galaxy's orbit and the time it entered the cluster.

2017-11-16
11:15
EMPIRE: Probing the dense, star-forming gas across entire disk galaxies
Maria Jesus Jimenez Donaire (ITA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
I will present results from a comprehensive and systematic study of the fraction of dense gas and its star formation efficiency as traced by several high critical density molecular lines. The results are based on the IRAM-30m large program EMPIRE, a ~500h survey that provides full maps of high critical density tracers like HCN or HCO+ across the entire star- forming disks of 9 nearby disk galaxies. My thesis work addresses one of the key goals of the survey: understanding how dense gas fractions and star formation efficiencies vary across and among galaxies. In particular, we find the star formation efficiency in the dense gas to vary systematically, at odds with a whole class of models based on Milky Way work. Another particular focus is to relate the fraction of star-forming gas and its ability to form stars to local ISM and dynamical conditions by studying their dependence with local conditions such as stellar surface density or ISM pressure. I furthermore present results of analyzing optically thin isotopologues (e.g. H13CN) and high signal-to-noise 13CO and C18O in EMPIRE and ancillary ALMA data, to constrain optical depths of high critical density tracers and abundance variations in our disk galaxies. Both quantities are crucial to understand the ISM density distribution.

2017-11-09
11:15
An agnostic look at the small-scale crisis in LCDM: insights from observations and simulations
Sebastian Trujillo-Gomez (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Galaxies constitute a powerful probe of the LCDM cosmological model at the smallest nonlinear scales, where no other robust observational probes are yet available. Using the number density of galaxies as a function of their rotation velocity (the galaxy velocity function, VF) we are able to confront theoretical predictions with observations of the nearby universe. Using the most sensitive currently available data we find a significant underabundance of structure at scales below the mass of the Small Magellanic Cloud. A complete accounting for baryonic effects including stellar feedback and UV photoheating does not resolve the discrepancy. After exploring a variety of solutions including warm or self-interacting dark matter (DM) as well as cosmological parameter variations we find that while the low-mass end of the VF is sensitive to DM particle physics, the massive end is a sensitive probe of cosmology. Recent hydrodynamic galaxy simulations offer solutions to the problem of the "missing dwarfs" but their detailed kinematics fail to reproduce those of the faintest observed galaxies. With the advent of the unprecedented HI surveys with the SKA and its precursors in the next few years, “observing” tightly constrained simulations will be key to understanding the influence of galaxy formation physics on the recovery of the potential from dwarf galaxy rotation curves and on their use as near-field cosmological probes.

2017-11-02
11:15
Using Globular Cluster Stellar Populations to Understand Galaxy Formation
Christopher Usher (Liverpool John Moores University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Globular clusters are important tools to help us understand how galaxies form and evolve. Globular cluster formation tells us about the conditions of extreme star formation while their survival from high redshift tell us about the processes of galaxy assembly. Being much brighter than red giant stars, globular clusters allow the stellar populations of galaxies to be studied at much greater distances, and thus a wider range of galaxy masses, environments and morphologies to be studied than can be with resolved stars. Using data from the WAGGS survey of massive star clusters in the Milky Way and its satellite galaxies and the SLUGGS survey of the globular cluster systems of massive early-type galaxies, I will talk about how we can measure the metallicities of globular clusters using the strength of the calcium triplet in integrated light. Using globular cluster metallicity distributions and the relationships between globular cluster colour and metallicity, I will present evidence that different galaxies with similar masses experienced different formation histories. I will compare these observations with the predictions of the E-MOSAICS cosmological simulations of the formation of globular cluster systems. I will also talk about how the E-MOSAICS simulations have allowed to us to understand how globular cluster colour distributions vary with globular cluster luminosity (the 'blue tilt').

2017-10-26
11:15
The (in)completeness of Gaia Data Releases
Ulrich Bastian (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
I will question the notion of "completeness" for a star catalogue, and briefly present how it relates to the concept of "selection functions". I will illustrate the point with examples from Gaia data, in the hope of triggering a lively on-the-spot discussion among the audience.

2017-10-19
11:15
How Superbubble-driven Outflows Shape Galaxies
Benjamin Keller (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
M* galaxies, with halo masses of about 10^12 Msun live in an interesting part of parameter space. Not only are they the "turnover" in the galaxy mass Schechter function, they also have the highest stellar mass (and baryon) fraction, very low bulge-to-disk ratios, and dominate the star formation of the epoch they live in. I will present the results of a sample of 18 cosmological M* galaxies, simulated using the state-of-the-art superbubble method for handling feedback from Type II supernovae. I will show that the key to obtaining a realistic stellar mass-to-halo-mass relation (SMHMR) is preventing the runaway growth of a massive bulge by driving outflows with large mass loadings. If this happens, SN feedback alone can no longer effectively drive outflows from the galaxy, and star formation becomes unregulated. This is a key piece of evidence that the peak of the SMHMR is due to the shut- down of SN regulation and the beginning of AGN regulation in more massive halos.

2017-07-27
11:15
Dense stellar systems, supermassive black holes, tidal disruptions
Rainer Spurzem (ARI, NAOC, KJAA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Many if not most galaxies contain supermassive black holes. In a gas free situation they are embedded in a very dense stellar cluster. The black hole grows through tidal disruption of stars and accretion of other black holes (stellar mass or other supermassive black holes after galaxy mergers). We study these processes with large direct N-body simulations including relativistic Post-Newtonian dynamics when needed. We find that tidal disruption rates can be enhanced in axisymmetric galactic nuclei; this effect is even more pronounced during a phase after galaxy mergers when the gravitational potential is non-axisymmetric and strongly varying in time. Recent results show that also the presence of a gaseous disk in the central galactic regions will strongly enhance tidal disruption rates by trapping stars on nearly circular orbits confined within the disk. A stellar disk forms and after the removal of the gaseous disk the nucleus has some common features with our own Galactic Center. Inspired by recent detections we will also discuss in some detail the expected gravitational wave radiation from black hole mergers on all mass scales in galactic nuclei and star clusters.

2017-07-20
11:15
Open star clusters in the Gaia era between DR1 and DR2
Siegfried Röser (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Open star clusters are representatives of the young stellar population in the Galactic disk. They are directly related to the locations of star formation. In the past decade we determined the astrophysical parameters of some 3000 open clusters in the solar neighborhood, based on membership determination from ground-based astrometry and photometry. This sample is, in general, complete up to a distance of 1.8 kpc from the Sun. However, for the oldest clusters - ages above 1 Gyr - we find an underdensity within 1 kpc from the Sun. The highly improved astrometric accuracy of the Gaia data releases enables new possibilities to detect those clusters. I will report on early successes, as well as on work in progress.

2017-07-13
11:15
High school students searching for asteroids with Pan-STARRS - hands-on educational activities with research-grade telescopes
Carolin Liefke (HdA/ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) performs an automated search for moving objects within the solar system. The International Astronomical Search Collaboration (IASC) educational program allows high school students to work with recent Pan-STARRS images to make their own discoveries of unknown asteroids. In this talk, I will present results and implications from almost seven years of the project and its unique approach to bring young minds in touch with real scientific data and analysis methods.

2017-07-06
11:15
Modelling the Milky Way Disc and Gaia Data
Andreas Just (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Many fundamental questions about the structure and evolution of the Milky Way’s thin disc are still under debate: What is its radial scale length? What is the star formation history in the solar neighbourhood? What is the reason for the dynamical heating (age-velocity dispersion relation) of the stellar sub-populations? Has the thin disc grown inside-out? Is radial mixing important to understand the dynamical and abundance properties of the disc stars? All these aspects are strongly correlated in any consistent physical model of the Milky Way. I will present the status of our analytic disc model and discuss the impact of the Gaia DR1 data on the local disc parameters. TGAS provides parallaxes and proper motions for more than a million stars in the solar neighbourhood. Combined with RAVE data, full 6-D param-eter space information is available for 250,000 stars allowing a stringent test of the local model. For the radial extension of the disc model additional large spectral surveys like APOGEE and GES are needed to infer the inside-out growth of the disc.

2017-06-29
11:15
Ultra-deep imaging in the nearby Universe
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Due to the limitations of current astronomical instrumentation and data reduction techniques, the ultra-low surface brightness (ULSB) universe – which lies over two orders of magnitude below that of the sky background – is one of the last niches that remain to be explored in observational parameter space. So far, the first pioneering observations using small telescopes and telephoto lens have revealed a wealth of stellar tidal streams and shells, diffuse stellar systems, and a possibly hitherto unknown type of galaxies (ultra-diffuse galaxies) whose properties may be different from those at brighter levels. In this talk, I will present the latest results of our deep exploration of the nearby universe and the prospects for further progress using both ground- and space-based observations.

2017-06-22
11:15
Cluster-Formation Conditions and Star Formation Relations
Genevieve Parmentier (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
In this contribution, the connections between cluster-formation conditions, star formation relations and star cluster evolution are investigated. Star formation relations present a diversity of slopes whose physical interpretation requires a careful understanding of the data they represent. From the quadratic star formation relation of molecular clouds of the Solar neighbourhood to the linear star formation relation of distant molecular clumps of the Galactic disc, I shall discuss what these relations tell us and, also, what they do not tell us. With the local quadratic star formation relation supporting the hypothesis that star clusters form in centrally-concentrated molecular clumps with a constant star formation efficiency per free-fall time, we shall see what the consequences for cluster-formation conditions are, and how cluster-formation conditions propagate through star cluster evolution.

2017-06-08
11:15
Gaia on the road to DR2
Michael Biermann (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
In September 2016 the first Gaia Data Release of the European space astrometry mission Gaia was published. At that time, Gaia-external data like the Hipparcos and TYCHO-2 catalogues had been used in order to allow for the early production of some parallaxes and proper motions. For April 2018, the second Gaia Data Release (Gaia DR2) has been announced. This release will be based on Gaia data only and will provide full Gaia astrometry and three-band photometry for the majority of all stars to be released. We present an outlook in this outstanding second Gaia Data Release and discuss its probable contents and expected uncertainties. The number of motions and parallaxes will rise from two million to the order of one billion, and the precisions and accuracies will significantly exceed those of Gaia DR1.

2017-06-01
11:15
Two-color microlensing observations and the abundance of Earth-mass planets
Markus Hundertmark (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Galactic gravitational microlensing events offer the unique opportunity to discover sub-Neptune mass planets orbiting stars located several kiloparsecs away from Earth. Moreover, the orbital radius is likely to be outside the snow line of water ice. Since the first reported microlensing planet in 2003, more than 50 exoplanets have been found with this method. Determining the planet mass and its orbital radius remains a challenge and often requires follow-up observations and further constraints. In this talk, I present new results of the RoboNet follow-up team and provide an outlook on how our recently approved LCO (Las Cumbres Observatory) key project will address the issue of determining the abundance of planets in the next three years.

2017-05-18
11:15
Deciphering the progenitors and explosion mechanisms of Type Ia supernovae
Markus Kromer (ITA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Type Ia supernovae are dramatic explosions that can outshine a whole galaxy for a short period of time. In the past two decades, they have been used as standardisable candles to measure the expansion history of the Universe. There is a wide consensus that Type Ia supernovae originate from thermonuclear explosions of carbon-oxygen white dwarfs in binary systems. However, the actual progenitor stars are still elusive and details of the explosion mechanism are not yet fully understood. In this talk, I will present how a synthesis of numerical explosion models and observational data can be used to shed light on these questions.

2017-05-11
11:15
CMB-lensing by nonlinear and non-Gaussian structures
Björn Malte Schäfer (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Weak lensing of the cosmic microwave background is a source of cosmological information from the redshift range z=3-5 which is difficult to probe by other means. The statistical detection of the statistical homogeneity-breaking by Planck in excess of 40 sigma has allowed the determination of cosmological parameters on the percent-level. In the interpretation of the signal one usually assumes Gaussian statistics of the deflecting large-scale structure, which is only approximately true due to non-Gaussianity generation in nonlinear structure formation. I show a few results that we obtained in relaxing the assumption on Gaussianity, both using analytical and numerical methods, and conceptual results concerning the CMB-lensing formalism.

2017-05-04
11:15
What does the Sun tell us about the Milky Way's chemical evolution?
Jan Rybizky (MPIA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
Elemental abundances of stars are the result of the complex enrichment history of their galaxy. The interpretation of observed abundances requires flexible modeling tools to explore and quantify the information about Galactic chemical evolution (GCE) stored in such data. I will present Chempy, a newly developed code for GCE modeling, representing a parametrized open one-zone model within a Bayesian framework. Unlike established approaches, Chempy can sample the posterior probability distribution in the full model parameter space and test data-model matches for different nucleosynthetic yield sets. As an illustrative application, I will show that interesting parameter constraints result from only the ages and elemental abundances of Sun, Arcturus and the present-day interstellar medium (ISM).

2017-04-27
11:15
Structural Properties of Edge-on Galaxies
Aleksandr Mosenkov (Ghent University and St. Petersburg State University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12 -14, Seminar Room 1
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Abstract
I will briefly review the main structural properties of edge-on galaxies, which can be derived via photometric decomposition analysis and radiative transfer modeling. I use observations from UV to far-infrared and sub-millimeter wavelengths, which allow us to study both stellar and dust properties. I consider edge-on galaxies of different morphologies: from early- to late-type spirals, including peculiar galaxies with polar rings and polar bulges, and galaxies with different structural features (disk warps and truncations, low-surface-brightness features, boxy/peanut-shaped bulges, and galaxies with very thin disks). For this study, the new software pipeline DECA for performing automated and manual disk-bulge decompositions and estimating galactic structural parameters was developed. DECA provides new capabilities to investigate large samples of galaxies and to obtain their general structural characteristics as well as to carry out a detailed analysis of various galactic features. I will discuss some well-known galaxy scaling relations with an attempt to link them to galaxy formation and evolution.

2017-04-20
11:15
The current status of the Southern African Large Telescope
Alexei Kniazev (SAAO Cape)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Abstract
The Southern African Large Telescope (SALT), a 10-m class telescope at the South African Astronomical Observatory near Sutherland, is the largest optical telescope in the southern hemisphere. SALT has now been in normal full science operations for more than five years. I will summarize and analyze the current status, achievements, and science done, in an attempt to place the project in the context of world-wide observational astronomy. After addressing technical challenges and streamlining operations, these first years of full operations at SALT have seen good and consistently increasing rates of completion of high priority observations and, in particular, very cost-effective production of science publications. Currently SALT clearly is a flagship astronomical facility in Africa, but what about its standing among its other peers?

2017-02-09
11:15
Python, TAP, and the Rest of the VO: PyVO!
Hendrik Heinl und Markus Demleitner (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The many useful service accessible through VO standards, and the Table Access Protocol (TAP) in particular, are a treasure trove to your research. If you're using Python, reaching into this trove has just become a lot easier thanks to the recent addition of TAP support to the astropy affiliated package PyVO done by ARI staff. In this talk, we show how this and other PyVO features let you access catalog data, query remote databases, integrate the results with your own code, and make your code play with other VO tools like TOPCAT or Aladin via SAMP. Even if you're not using Python yet: you can take this opportunity to get a taste of what you might be missing.

2017-02-06
11:15
Stellar clusters in the Gaia-ESO survey
Angela Bragaglia (INAF - Osservatorio Astronomico Bologna)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The Gaia-ESO survey is a large, public survey with FLAMES at the ESO VLT using 300+ nights over more than 5 years. Gaia-ESO is obtaining high-resolution spectra of about 100000 stars of all Galactic populations to derive kinematics, metallicity and detailed chemical abundances to complement the Gaia mission. In particular, Gaia-ESO is targeting open clusters of all ages, from star forming regions to mature and old object. The goal is to cover the metallicity / age / position / mass distribution of clusters to follow their formation, evolution and dissolution and to use them as probes of the Galactic disk. I will present the survey status, focusing on the stellar clusters.

2017-02-02
11:15
A high-resolution study of massive YSOs in the Magellanic Clouds
Jacob Ward (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The effects of metallicity on the physical processes that govern the formation of massive stars are undoubtedly significant yet remain poorly understood. At distances of ~50 and ~60 kpc, the sub-solar metallicity Magellanic clouds present a unique opportunity to study star formation on small spatial scales in environments which differ significantly from those of our own Galaxy. Using integral field spectroscopic data obtained with VLT/SINFONI we have investigated the properties of 19 Spitzer selected massive YSOs in the Small Magellanic Cloud and 8 in the Large Magellanic Cloud at sub-parsec resolutions for the first time. A detailed comparison of these objects with a large sample of comparable Galactic sources has revealed significant differences in the properties of YSO in the lower metallicity Magellanic Clouds and hints towards a significant relation between metallicity and accretion rates.

2017-01-26
11:15
Insights into the chemical composition of the metal-poor Milky Way halo globular cluster NGC 6426
Michael Hanke (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Recent photometric and spectroscopic studies indicate that globular clusters generally are composed of multiple populations. Hence, they radically break with the long-lasting paradigm of considering globular clusters as prime examples for so-called simple stellar populations. Seeking answers to questions about the formation mechanisms of such multiple populations in globular clusters raises the need for detailed spectroscopic analyses of the chemical composition of these stellar systems. In this talk I will present our recent chemical abundance study of four giant stars associated with NGC 6426, one of the most metal-poor Galactic globular clusters known. First, I will briefly introduce EWCODE, a tool we developed in order to accurately and reproducibly determine equivalent widths from spectra like our high-resolution MIKE data. After spending some time discussing the methods applied to derive stellar parameters and subsequently chemical abundances, I will show our results for 22 chemical elements. Among others, we found imprints from hypernova nucleosynthesis as well as from a yet unknown lighter element primary process (LEPP). Finally, tentative evidence for the existence of two chemically distinct subpopulations in NGC 6426 will be discussed.

2017-01-19
11:15
The Influence of Galaxy Environment on the Stellar Initial Mass Function of Early-Type Galaxies
Giulio Rosani (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
We investigated whether or not the stellar initial mass function of early-type galaxies depends on their host environment. For this purpose we selected a sample of early-type gal-axies from the SPIDER catalogue (La Barbera et al. 2010), obtained information on their environment through the group catalogue of Yang et al. (2007), and used the optical SDSS spectra of these galaxies to determine the line strength of IMF sensitive indices, from which we derive the IMF slope. To reach a high enough signal-to-noise ratio we stacked the spectra in velocity dispersion (σ0) bins, on top of separating the sample by galaxy hierarchy and host halo mass, a proxy of their environment. We then compared the observed line strengths with predictions from the MIUSCAT/EMILES synthetic stellar population models to constrain the best fitting slope of the IMF (Γb) in the stacked spectra. We also explored how the results depend on different sets of isochrones and the way elemental abundance ratios are fitted. We find that Γb increases with σ0 and becomes bottom-heavy at high σ0. No dependence of Γb on environment or hierarchy is detected, except possibly when low σ0 centrals in low and high mass haloes are compared. This feature is likely an effect of the age-dating technique, but further investigation is required.

2017-01-13
11:15
The Gaia-ESO Survey: Galactic evolution of sulphur and zinc
Elisabetta Caffau (Observatoire de Paris)
ARI Institute Colloquium
ARI Mönchhofstrasse 12 - 14, Seminar Room 1
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Abstract
We exploited the Gaia ESO Survey (GES) data to study the behaviour of sulphur and zinc abundances of a large number of Galactic stars, in a homogeneous way. We confirm the results from the literature that sulphur behaves as an alpha-element. We find a large scatter in [Zn/Fe] ratios among giant stars around solar metallicity. The large scatter in zinc among giant stars and the difference between dwarfs and giants has no definite explanation.

2017-01-12
11:15
The Blazhko effect and additional periodicity among RR Lyrae stars in the Galactic bulge
Zdenek Prudil (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Currently, it is generally believed that half of the fundamental mode RR Lyrae stars exhibit modulations of this light curves known as the Blazhko effect. The Blazhko effect produces additional equidistant peaks in the vicinity of the basic pulsation frequency components in frequency spectra. We looked for such patterns among fundamental mode RR Lyrae stars in the Galactic bulge. We found that in a total sample of 8 282 RRab stars 40.3 % of the stars exhibit the Blazhko effect. The number of Blazhko stars we identified is 3341, which is the largest sample ever studied implying the most relevant statistical results currently available. Blazhko stars with modulation occur preferentially among RR Lyrae stars with shorter pulsation periods in the Galactic bulge. Fourier amplitude and phase coefficients based on the mean light curves appear to be substantially lower for Blazhko stars than for stars with an unmodulated light curve. In addition, we discovered a new group of double-periodic stars in the OGLE Galactic bulge photometry. In 38 stars identified as fundamental mode RR Lyrae and 4 classified as the first-overtone RR Lyrae, we detected additional shorter periodicity. At this point, we cannot explain the nature of the additional periodicity.

2016-12-22
11:15
What is Science?
Joachim Wambsganss (ARI/ZAH)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The topic of this talk is based on my year as a „Marsilius Fellow“ in the Marsilius-Kolleg of Heidelberg University. The idea behind the Marsilius Kolleg is to bridge the gap between sciences and humanities through meetings, discussions and joint projects, which in turn promote understanding and cooperation between the sciences and the humanities. Founded in 2007, the Marsilius Kolleg is a central component of Heidelberg University’s successful proposal granted by the Excellence Initiative, launched by the federal and state governments of Germany. My Marsilius project was entitled „Wissen, Glaube(n), Aberglaube“ [knowledge, belief, superstition], together with a sociologist and a historian/theologist we addressed different facets of this theme. At this year's ARI christmas colloquium I will discuss various aspects of the simple-sounding question: "What is science?“, based largely on the book "Systematicity – The Nature of Science“ by Paul Hoyningen-Huene. I will try to define science and to demarcate it from non-science and pseudo-science.

2016-12-15
11:15
The MESSIER satellite: lifting the veil on the ultra-low surface brightness universe
David Valls-Gabaud (Observatoire de Paris)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The S-class MESSIER satellite has been designed to explore the extremely low surface brightness universe at UV and optical wavelengths. The two driving science cases target the mildly- and highly non-linear regimes of structure formation to test two key predictions of the LCDM scenario: (1) the detection of the putative large number of galaxy satellites, and (2) the identification of the filaments of the cosmic web. The satellite will drift scan the entire sky in 6 bands covering the 200-1000 nm wavelength range to reach the unprecedented surface brightness levels of 34 mag/arcsec^2 in the optical and 37 mag/arcsec^2 in the UV. Many important secondary science cases will result as free by-products and will be discussed in some detail, such as the luminosity function of galaxies, the contribution and role of intracluster light, the fluctuations of the cosmological background radiation at UV and optical wavelengths, the molecular hydrogen content of galaxies at z=0.25, time domain studies of supernovae and tidal disruption events, the chemical enrichment of the interstellar medium through mass loss of red giant stars and the accurate measure of the BAO scale at z=0.7 with over 30 million galaxies detected in Lyman-alpha at this redshift. It will provide the first space-based reference UV-optical photometric catalogue of the entire sky, and synergies with GAIA, EUCLID and WFIRST will also be discussed. Technical issues will likewise be addressed for possible improvements on the current design.

2016-12-12
11:15
In-situ IMF at z~2
Themiya Nanayakkara (Swinburne University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, ARI Seminar Room 1
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Abstract
The development of sensitive Near Infra-Red instruments has made it possible to study galaxy properties at z~2, just 3Gyr after the Big Bang. This is expected to be the time period where galaxies are actively star forming and evolving rapidly to form the massive galaxies that are observed in our local neighbourhood. ZFIRE is a survey, which utilises the MOSFIRE instrument on Keck telescope over 18 nights to study properties of ZFOURGE selected mass complete galaxies in rich environments at z~2. In my talk I will present results of the first ever attempt to constrain the Initial Mass Function (IMF) of galaxies at these redshifts using a cluster and a field sample. We have investigated the degeneracy between the star formation histories and the IMF to make strong constrains on the stellar mass distribution of these galaxies using synthetic stellar spectra. I will focus on the role of dust, star-bursts, stellar rotation, binaries, and metallicity on determining observed galaxy properties at z~2 to address whether ZFIRE results favour the canonical concept of a universal IMF.

2016-12-08
11:15
A chemical study of the old open cluster M67
Clio Bertelli Motta (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Open clusters are known to be the birthplace of most field stars. As such they can be treat- ed as benchmark for studies of stellar and Galactic evolution. Because of its unusually advanced age (3-4 Gyr), its proximity to the Sun (800-900 pc), and its Sun-like chemical composition, M67 has become one of the most extensively studied open clusters in the Milky Way. Many of M67's sub-giant branch, giant branch, and red clump stars were spectrometered in the APOGEE survey. Combining proper motions, radial velocities, colour-magnitude properties, and metallicities, we carried out a careful membership anal- ysis of the stars observed within the projected diameter of M67. We then investigated the abundances of several elements in cluster member stars as measured by the APOGEE pipeline ASPCAP. Along the sub-giant and giant branch we detected effects of the first dredge-up, as the products of the CNO cycle in the hydrogen-burning shell are brought to the surface by convection in the envelope, thus changing the surface abundance of [C/Fe] and [N/Fe]. We compared the abundances of these elements with the stellar evolution models by Salaris et al. (2015), which we find to be in very good agreement with the mea- sured [C/N] abundances of the M67 red giants, thus posing a strong constraint on the calibration of [C/N] as an age indicator for field stars.

2016-12-01
11:15
The Universal Physics of Cluster Formation and Survival from Local Galaxies to High Redshift
Diederik Kruijssen (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
I will discuss how the past few years have witnessed a major change in our understanding of cluster formation and destruction. The classical picture of infant mortality (all stars form in clusters of which 90-95% subsequently dissolve due to gas expulsion) has been replaced by a more general model, in which star formation takes place across the full density spectrum of the ISM and only the highest-density peaks reach the star formation efficiencies necessary to become a bound cluster. While the implications of this model are that both the fraction of star formation occurring in bound clusters and the maximum cluster mass depend on the galactic environment, the underlying physics are universal. The same applies to the destruction of stellar clusters, which is now known to be dominated by tidal interactions with dense gas structures in the natal galaxy disc, rather than internal processes such as gas expulsion or evaporation. I will discuss these developments and present new observational tests of these ideas, from the Central Molecular Zone of the Milky Way to nearby dwarf, spiral, and starburst galaxies. The good agreement between theory and observations motivates an application to cluster formation and destruction across cosmic time. I will present a new model that considers the local-Universe physics of cluster formation and destruction in the context of galaxy formation, demonstrating that globular clusters are the natural outcome of regular star and cluster formation in ‘normal’ high-redshift galaxies. This unified model for cluster formation across cosmic time provides important constraints for scenarios aiming to explain the multiple stellar populations in globular clusters, and makes strong predictions for upcoming observations by ALMA, Gaia, JWST and the E-ELT.

2016-11-24
11:15
Revealing the secrets of Westerlund 2 - A young massive star cluster observed with the Hubble Space Telescope
Peter Zeidler (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
We present a detailed study of the young massive star cluster Westerlund 2 (Wd2) using a high-resolution multi-band survey observed with the Hubble Space Telescope.  With an estimated mass of ~4×104 M⨀, Wd2 is one of the youngest (~1 Myr) massive star clusters in the Milky Way and contains a rich pre-main-sequence population. We show that the mass accretion rate of the pre-main-sequence stars is reduced in proximity to the cluster’s massive O stars, which are eroding the circumstellar disks of the low-mass stars. We find that Wd2’s mass function is steeper than the canonical slope of the initial mass function. The cluster shows mass segregation, which is likely primordial given its young age.

2016-11-17
11:15
Milky Way archaeology from Cepheids
Bertrand Lemasle (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Cepheids are excellent tracers of young stellar populations (20-300 Myr). They are famous for providing very accurate distances, their ages are also very well known and, more generally, their physics is well understood. Moreover, their spectra show well-defined absorption lines leading to reliable abundances for numerous elements. For all these reasons they have been used for almost 30 years to study the Milky Way chemical evolu- tion. In this talk I will discuss specific challenges related to the analysis of Cepheid spectra and present some recent results using Galactic Cepheids. I will also say a few words about future perspectives in terms of observing facilities and analysis techniques.

2016-11-10
11:15
Infrared Astrometry - the science case, the technological challenges,the future missions
Wolfgang Löffler (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
This talk will begin with a presentation of the general science case for infrared astrometry. Focussing on the special case of the Galactic centre, I will review the proposed Small-JASMINE infrared astrometry mission and discuss its scientific and technological challenges. The state of the art and possible developments in the fields of CMOS detectors and cooling are identified as crucial issues for future astrometry missions in the infrared. The GaiaNIR technology proposal is presented as a way forward regarding the CMOS detectors.

2016-11-03
11:15
Proxima Centraui b: Our next door neighbour
Yiannis Tsapras (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The Pale Red Dot campaign (20 Jan-31 Mar 2016) was an international search for a terrestrial exoplanet around Proxima Centauri. Radial velocity measure- ments were obtained with the HARPS spectrograph on ESO's 3.6m telescope at La Silla and simultaneous photometric observations were collected with the Las Cumbres Observatory (LCO) telescope network and the ASH2 0.4m tele- scope in Chile. The project included a signi cant outreach component which aimed to educate, inspire and inform the public througout the campaign. I will present a summary of the campaign and report on the discovery of planet Proxima Centauri b.

2016-10-27
11:15
The Magellanic Stream: Circumnavigating the Galaxy
Elena D'Onghia (Department of Astronomy, University of Wisconsin-Madison)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The Magellanic Clouds are surrounded by an extended network of gaseous structures. Chief among these is the Magellanic Stream, an interwoven tail of filaments trailing the Clouds in their orbit around the Milky Way. When considered in tandem with its Leading Arm, the Stream stretches over 200° on the sky. The Stream is thought to represent the result of tidal interactions between the Clouds and ram-pressure forces exerted by the Galactic corona, and its kinematic properties reflect the dynamical history of the pair of dwarf galaxies closest to the Milky Way. The Stream is a benchmark for hydrodynamical simulations of accreting gas and cloud/corona interactions. If the Stream survives these interactions and arrives safely in the Galactic disk, its cargo of over a billion solar masses of gas has the potential to maintain or elevate the Galactic star-formation rate. I review the current state of knowledge of the Stream, physical conditions, origin, and fate. I also review the dynamics of the Magellanic System, including the proper motions and orbital history of the Large and Small Magellanic Clouds, the first-passage and second-passage scenarios, and the evidence for a Magellanic Group of galaxies.

2016-10-20
11:15
Fossils of galaxy evolution: what do the Milky Way satellites tell us?
Eva Grebel (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The Milky Way is surrounded by a large entourage of satellites covering a wide range of luminosities, masses, and star formation histories, and newly discovered ultra-faint satellites continue to be added. All of these galaxies are close enough to be resolved into stars, allowing us to conduct highly detailed "galactic archaeology" of their star formation histories, chemical evolution, and dynamical evolution. All of these satellites contain old, metal-poor stellar populations, providing unique insights in the early stages of star formation and chemical enrichment in small dark matter halos at high redshift. The current satellites are the survivors of a once much more numerous population of since accreted Galactic building blocks that contributed to the build-up particularly of the Galactic halo. Hence their characterization holds important clues on the evolutionary history of our Milky Way itself.

2016-07-28
11:15
Not-so-simple stellar populations in nearby, resolved massive star clusters
Richard de Grijs (Kavli Institute for Astronomy and Astrophysics, Peking University, China)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Until about a decade ago, star clusters were considered "simple" stellar populations: all stars in a cluster were thought to have similar ages and the same metallicity. Only the individual stellar masses were thought to vary, in essence conforming to a "universal" initial mass function. Over the past decade, this situation has changed dramatically. Yet, at the same time, star clusters are among the brightest stellar population components and, as such, they are visible out to much greater distances than individual stars, even the brightest, so that understanding the intricacies of star cluster composition and their evolution is imperative for understanding stellar populations and the evolution of galaxies as a whole. I will discuss my group's recent progress in this context, with particular emphasis on the properties and importance of binary systems, the effects of rapid stellar rotation, and the presence of multiple populations in Local Group star clusters across the full age range. Our most recent results imply a reverse paradigm shift, back to the old simple stellar population picture for at least some intermediate- age (~2 Gyr-old) star clusters, which opens up exciting avenues for future research efforts.

2016-07-21
11:15
Dynamical interaction of Supermassive Black Holes with the surrounding stellar system
Taras Panamarev (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Galactic nuclei are perfect laboratories for studying physical processes in the vicinity of a supermassive black hole and for exploring the impact of the massive black hole on its stellar and interstellar environment. High resolution observations of the Milky Way nucleus, which is very quiet, provide detailed and specific information that can be compared to numerical models. Our model for the dynamical evolution of a galactic nucleus takes into account the formation and evolution of binaries as well as stellar evolution. We model the tidal disruption and accretion of stars onto the central black hole by a simplified model.In this talk I will present some results from the ongoing simulation of the Galactic Centre.

2016-07-14
11:15
Gravitational Wave Astrophysics - Simulations and Observations
Rainer Spurzem (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
I will present an overview of black holes of different origins and masses in the universe and their role as sources of gravitational waves, across the entire wavelength range of future gravitational wave astrophysics. The dynamical evolution of these sources, their detailed gravitational wave emission, and methods for detection will be discussed - this topic ranges from the dynamics of supermassive black holes in galactic nuclei to globular cluster evolution (with stellar mass black holes). The method of Post-Newtonian dynamics is used both for dynamical evolution models and detailed prediction of gravitational wave signals from black hole binaries. We detect objects very similar to the famous recent gravitational wave source GW150914 in our simulated data and show their emitted gravitational waveforms match the observed one; in addition to that we know the detailed dynamical history of the object. If time allows, the necessary high performance supercomputing software and hardware needed to do our simulations, current and future, will be discussed.

2016-07-07
11:15
Mapping the Magellanic Clouds in 3D to trace their interaction history
Smitha Subramanian (Kavli Institute Astronomy and Astrophysics Beijing)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The Magellanic system (located at a distance of ~ 55 kpc) which comprises of the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), the connecting Magellanic Bridge (MB), the leading arms and the Magellanic Stream (MS), is one of the nearest examples of an interacting system in the local universe and an excellent template to study galaxy interactions using stellar populations. Based on the revised proper motion estimates from the HST, it is more or less well established that the formation and the evolution of the Magellanic system is mainly driven by the mutual interaction of the Magellanic Clouds. In this talk, I will present some of our work based on stellar populations (using optical and NIR photometric data) to understand the 3D structure and interaction history of the Magellanic Clouds.

2016-06-30
11:15
Exploring the Gaia Data with TOPCAT
Hendrik Heinl (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
With the first Gaia Data Release by the end of summer 2016 astronomers will face the challenge of data intensive science. The custom workflow of downloading whole catalogs and running the data through a local pipeline will change due to the sheer amount of data: downloading several TB of data will take at best several hours, in average days and also processing the data with local pipelines will be time and resource consuming. To solve these bottleneck problems, astronomers should be enabled to select only that subset of the data, they are actually interested in. The concept is: instead of bringing the data to the code, bring (parts of) the code to the data. For this, the Virtual Observatory developed the Table Access Protocol (TAP) that uses the Astronomical Data Query Language (ADQL) to bring code to the data and select subsets. In this talk I will present how to remotely explore catalogs and select a subset of the data TOPCATs TAP interface. In particular I will demonstrate how to perform crossmatches between subsets of the Gaia modelled catalog published via the ESA archive with surveys like 2MASS and SDSS.

2016-06-29
16:15
Modelling of individual globular clusters using N-body simulations
Holger Baumgardt (Univ. Queensland, Australia)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
We have determined the masses and mass-to-light ratios of 50 Galactic globular clusters by comparing their velocity dispersion and surface brightness profiles against a large grid of 900 N-body simulations of star clusters of varying initial concentration, size and central black hole mass fraction. Our models follow the evolution of the clusters under the combined effects of stellar evolution and two-body relaxation allowing us to take the effects of mass segregation and energy equipartition between stars self-consistently into account. Our results show that the mass-to-light ratios of Galactic globular clusters agree well with the expected M/L ratios for standard Kroupa or Chabrier mass functions. Contrary to recent literature claims we do not find a decrease of the average mass-to-light ratio with metallicity. The surface brightness and velocity dispersion profiles of most globular clusters are incompatible with the presence of intermediate-mass black holes in them. The only exception is Omega Cen, where the velocity dispersion profile provides strong evidence for the presence of a 40,000 Msun IMBH in the centre of the cluster.

2016-06-23
11:15
Young massive clusters and the origin of multiple stellar populations in globular clusters.
Ivan Cabrera-Ziri (ESO, Garching)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Studies during the last decade have revealed that nearly all Globular Clusters (GCs) host multiple populations (MPs) of stars with a distinctive chemical patterns in light elements. No evidence of such MPs has been found so far in lower-mass (<∼ 10^4 M⊙) open clusters nor in intermediate age (1–2 Gyr) massive (> 10^5 M⊙) clusters in the Local Group. Young massive clusters (YMCs) have masses and densities similar to those expected of young GCs in the early universe, hence they are ideal places to test GC formation theories. For this talk, first we will review the constraints on GC formation scenarios obtained from studies of YMCs. Then we will present a new technique devised to look for chemical anomalies, i.e. MPs, within YMCs using their integrated light, and show the results of this technique. And finally we will discuss the recent results by Li et al. (2016), who claimed to have found evidence for multiple generations of stars in three intermediate age clusters in the Large and Small Magellanic Clouds (LMC/SMC). We show that the claimed 'younger stars' are part of the field population, and not likely to be physically associated with the clusters. We conclude that all proposed models for the origin of MPs are at odds with observations of YMCs.

2016-06-16
11:15
Constraints on the stellar IMF of early-type galaxies from optical and NIR spectral features
Francesco La Barbera (INAF, Naples)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Understanding the stellar initial mass function (IMF) is a key aspect to obtain a complete picture of galaxy formation and evolution. In the last years, we have carried out a system-atic census of the IMF in the unresolved stellar populations of (massive) early-type galaxies (ETGs), using optical and NIR spectroscopy from different surveys (e.g. SDSS, CALIFA) and dedicated observing programmes (OSIRIS@GTC, XSHOOTER@VLT). I will show results on a non-universal IMF in ETGs (pointing to an excess of low-mass stars in high-relative to low-mass galaxies) and current constraints on the driver behind observed variations in the stellar IMF.

2016-06-10
11:15
How can we use globular clusters to determine galaxy mass?
Gretchen Harris (University of Waterloo)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
We now know that virtually all galaxies contain a population of globular clusters (GCs) with remarkably similar properties (age, luminosity, metallicity...). Because GCs are luminous and often found at large galactocentric distances it is enticing to try and use their kinematic properties to determine galactic mass and mass distribution. To date there are more than two dozen GC systems with 100 or more known cluster velocities. I will briefly present the data and some of the simple results, leading to discussion of how well they can do this job

2016-06-09
11:15
Globular Cluster Systems, Galaxy Halos, and Dark Matter
Prof. Bill Harris (McMaster University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Galaxies are embedded in dominant dark-matter halos, but it has been difficult to find any visible stellar population that correlates in a simple way with DM halo mass. Systems of globular clusters have now been measured in hundreds of galaxies of all types, ranging from dwarf spheroidals up to supergiant Brightest Cluster Galaxies. This empirical evidence reveals an extremely strong and (surprisingly) near-linear relation between the total halo mass of a galaxy and the total mass in all its globular clusters -- a relation that is unlike any other known stellar population. Why? I will characterize what we know observationally about this correlation and describe early stages of model interpretation.

2016-06-02
11:15
Looking for Exoplanets around Giant Stars: the Past and the Future
Sabine Reffert (LSW)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Our Doppler survey of giant stars at Lick Observatory started in 1999 and ended suddenly in 2011, when the iodine cell which we were using for precise radial velocity determination was damaged. Nevertheless, interesting discoveries still continue to drop out of this rich data set (in total almost 20 000 spectra for a sample of 373 giant stars). I will summarize the most interesting results obtained so far, including individual systems (e.g. planets in spectroscopic binaries, multi-planet systems) as well as statistical results (planet occurrence rate as a function of mass and metallicity). In the second part of my talk, I will present our plans for the future. Doppler monitoring of the sample is expected to resume soon from Koenigstuhl, thanks to the newly-built Waltz Spectrograph. The extension of the survey time span to more than 20 years will allow for the characterizaton of much longer orbits than usually probed by similar surveys, and I will show that we have some very good indications for planetary as well as stellar companions with periods of decades in our sample.

2016-05-19
11:15
Accessing the Gaia catalog at ARI
Grégory Mantelet (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
tba

2016-05-17
11:15
Unfolding the secrets of the Galactic bulge
Istvan Dekany (University Chile, Santiago)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Unraveling our Galaxy's present structure and formation history is of profound importance in astrophysics because it holds key implications for galaxy evolution in general. The Milky Way is the only galaxy for which the most fundamental challenge in astronomy, the distance problem can be overcome, enabling us to dissect it for scientific scrutiny. The long-standing challenge, that we have to study it from inside, yet we wish to see it from a perspective like if it was an external galaxy, is now mitigated by large surveys. The Galactic bulge, although notoriously hard to study, could provide us with critical insights on the Milky Way's history, since it hosts an array of stellar populations, that have witnessed various key epochs of Galaxy evolution. In my talk, I will review recent significant advances in solving the problem of the bulge, with an emphasis on leveraging optical and near-infrared time-domain observations of its pulsating stars. The emerging picture is a composite bulge, hosting an ancient spheroid and a more prominent pseudobulge that formed via disk instabilities, and an even younger inner disk that has undergone recent stellar formation.

2016-05-12
11:15
Analyzing Quasars with Gaia
Svea Proft (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
ESA's cornerstone mission Gaia, offering the first data release in summer 2016, is primarily designed to investigate our galaxy. In this talk, I discuss the opportunities and limits in investigating quasars with the Gaia satellite by studying Gaia&#8217;s low- and high-resolution quasar spectra for different magnitudes and redshifts. In particular, I focus on variability observed in quasar spectra and their prospects to measure with Gaia.

2016-05-03
11:15
The complex stellar populations of globular clusters: how did they form?
Maurizio Salaris (Liverpool John Moores University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The last 10 years have witnessed a huge change in our understanding of globular clusters. We now know that (almost) each globular cluster hosts a complex stellar population displaying well defined chemical abundance patterns. The main ideas put forward to explain the presence of multiple stellar populations in individual clusters will be discussed, together with a number of observational facts that challenge these scenarios and point (maybe) to the need of alternative theories.

2016-04-28
11:15
Formation and Evolution of Giant Molecular Clouds: Gravity or Turbulence?
Mordecai-Mark Mac Low (ITA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Star formation in galaxies is observed to occur in molecular clouds. However, the origins and evolution of these clouds remains unclear, impeding the construction of a predictive theory of star formation. Indeed, the necessity of molecules for star formation has come into question, as it becomes clear that their formation may just be a side effect of star formation, correlated with gravitational collapse but not causing it. This seems consistent with the structuring of star-forming galaxies by gravitational instabilities that drive the formation of spirals and bars. In this talk I present kiloparsec-scale, well-resolved, MHD simulations of the stratified, supernova-driven, interstellar medium performed with the Flash code. I use the results of these simulations to argue that gravitational collapse rather than any sort of turbulent equilibrium determines the dynamics of molecular clouds, with a focus on the relationships between size, velocity dispersion, and surface density of the clouds.

2016-04-21
11:15
A peek into the nearby galaxy clusters' past
Thorsten Lisker (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The nearby galaxy clusters are not alike in terms of their galaxy content, their presumed evolutionary state, and their large-scale environment. In our ongoing work we are exploring various possibilities of utilizing the abundant dwarf galaxies to probe the clusters' accretion histories and the buildup of their galaxy populations. Stellar population properties can tell when star formation was quenched; the amount of rotational support indicates whether tidal heating has acted long enough to change the dwarfs' internal structure; and the phase-space distribution helps to distinguish between the long-standing galaxy population and new arrivals. At the extreme ends of galaxy parameter space, ultra-diffuse galaxies seem to somehow withstand the cluster's tidal field, while ultra-compact dwarfs are said to be messengers from early galaxy disruptions. Deciphering the message they carry is the challenge we face. The data and analyses I am going to present for a variety of low-mass galaxies i n the Fornax, Virgo, and Perseus clusters are important steps towards achieving this goal.

2016-04-14
11:15
Bar formation in galaxies with cuspy bulges
Evgeny V. Polyachenko (Institute of Astronomy RAS, Moscow)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
We suggest a solution for a problem of bar formation in galaxies with cuspy bulges, e.g., the Milky Way galaxy. The problem is connected with the presence of the so called inner Lindblad resonance (ILR), which exists for any possible pattern speed of the bar. From a theoretical point of view, the ILR prevents bar formation, although N-body experiments show a bar development by the usual bar instability during 3 Gyr. A possible reason is the initial disc thickness, to which the pattern speed and orbit resonances are very sensitive. When a disc particle orbit reaches a height above the Galactic plane comparable to the radial distance, the motion is no longer periodic in radial direction. As a consequence, for radii of the order of the characteristic disc height, the radial frequency is ill defined and the ILR is smeared out. Accounting for the disc thickness in a toy model allows us to reproduce the bar mode by a global mode analysis in the framework of linear perturbation theory.

2016-04-13
14:15
Instabilities in the Galactic disc: the role of spectroscopic surveys and current developments
Arnaud Siebert (Univ. Strasbourg)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Galactic spectroscopic surveys in the Galactic disc, such as RAVE and SEGUE, have recently exposed departures from a pure axisymmetric and stationary model. If the origin of these departures and how it affects our current understanding of the Milky Way is yet unclear, significant efforts are underway to clarify the situation. In this talk, I will present the detections of non-axisymmetric signatures in the bulk motion of stars and describe the different physical models that can eproduce these observations. I will conclude by presenting how the forthcoming large surveys will help remove our current limitations and will permit to disentangle the various models.

2016-04-06
11:15
The Death of Cluster Galaxies: Mergers, Strangulation, and More
Rukmani Vijayaraghavan (University of Virginia)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Groups and clusters are harsh, hostile environments for their host galaxies. Gas rich, star forming galaxies are stripped of their gas, dark matter, and stars before eventually being transformed to passive and gas poor galaxies. The process of transformation can be dramatic and varied; mergers between clusters or groups and clusters can accelerate these galaxy transformation processes via various pre-processing and post-processing mechanisms. Galaxy transformation processes do not behave as simply as expected; a significant fraction of cluster galaxies retain hot compact coronae that resist stripping, form stars in stripped tails, and have AGN. Galaxies themselves can generate turbulence and amplify magnetic fields in the intracluster medium in the process of being stripped. Using numerical simulations, I will provide some physical insight into the dramatic death of galaxies in group and cluster environments.

2016-02-17
14:15
Structure of the ISM in the extreme environment of LMC/30 Doradus.
Mélanie Chevance (CEA Saclay)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
I will present a far-infrared (FIR) view of the extreme star-forming region 30 Doradus in the Large Magellanic Cloud (LMC). The 30 Doradus region offers the best laboratory to examine the interplay between stellar activity and a metal-poor interstellar medium (ISM). The main stellar source of radiation is provided by the closest example of a super star cluster, R136, while the ISM is half-solar metallicity. The proximity of 30Dor (50kpc) makes it possible to study gas and dust over large scales in this dramatic environment. The new Herschel/PACS and SPIRE/FTS observations of far infrared (FIR) fine structure lines, combined with Spitzer/IRS spectroscopic maps, are used to constrain the physical conditions in the photo-dissociation regions (PDR) with the Meudon PDR code (Le Petit et al., 2006). This allows us to construct a comprehensive, self-consistent picture of the density, radiation field, and ISM structure. We quantify the effect of intense radiation field on this low metallicity ISM. In particular, we bring constraints to the fraction of molecular dark gas not traced by CO, the so-called “CO-dark gas”. This spatially resolved study of the star formation activity and the PDR/molecular gas aims at identifying and calibrating the specific diagnostic tracers such as [CII], [CI] and CO that can be used to characterize unresolved high redshift galaxies.

2016-02-11
11:15
Tracing feedback in massive star-forming regions with integral field spectroscopy
Anna McLeod (ESO Garching)
ARI Institute Colloquium
Astronomisches Rechen-Institut, Mönchhofstr. 12-14, Seminar Room 1
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Abstract
Throughout their entire life, massive stars deeply affect their surroundings: via outflows when they first form, via strong stellar winds and powerful ionising radiation later on, and via energetic supernovae explosions when their life ends. These effects can be followed and quantified by simulating their formation and evolution within molecular clouds, but observationally speaking, the quantification of the feedback mechanisms is very difficult. The difficulties can, however, be overcome in the era of integral field spectroscopy (IFS). The results of recent observations of a variety of structures and environments will be shown, carried out with the IFS MUSE and KMOS at the VLT, operating in the optical and near-infrared respectively. In our observational campaign pillar-like structures, HII regions and massive star forming regions like W49 and W33 were targeted, and photo-evaporative flows, ultra-compact HII regions, expanding bubbles and accretion features detected. With the unprecedented combination of spatial and spectral resolution of IFS, we were also able to detect and classify the possible massive O-stars responsible for the feedback in these regions. Ultimately, the quantification of massive star formation feedback in terms of comparing results in several regions will be discussed.

2016-02-04
11:15
STARDISK Project: The effect of the accretion disk on the central star cluster dynamics in AGNs
Bekdaulet Shukirgaliyev (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
An active galactic nucleus is typically composed of a super-massive black hole, a gaseous accretion disk and a compact star cluster, so that its activity can be represented as the result of the interaction among these three subsystems. We investigate the dynamical interaction of a central star cluster surrounding a super-massive black hole with a central accretion disk. The dissipative force acting on the cluster stars leads to an asymmetry in the phase space distribution of the central cluster due to the rotating accretion disk. I will present some results of the Stardisk model, where we see changes in the density profile of the central star cluster due to influence of the rotating accretion disk.

2016-01-28
11:15
Gaia's star clusters: A pretty little story - and the importance of using variance/covariance quantities
Ulrich Bastian (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The first Gaia data release is approaching. It is planned for summer this year. The data reduction is almost complete, and the validation of the outcomes is in full swing. I will tell a little story about a few star clusters as seen by Gaia, and then will show a caveat example illustrating that in addition to published parallaxes and proper motions, any putative Gaia user must always also take their (also published) uncertainties and correlations into account.

2016-01-21
11:15
Gamma-Ray Astrophysics from Black Holes to Galaxies
Frank Rieger (ITA / MPIK)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Very High Energy Gamma-Ray Astronomy has been substantially progressing over the last couple of years. In this talk I will highlight the fundamental physics potential of some recent results including the extreme gamma-ray variability characteristics seen in extragalactic sources that point to an origin of the radiation close to the supermassive's black hole horizon. Insights into the associated non-thermal processes are expected to be of relevance for understanding the activity of the central black hole in our own Galaxy.

2016-01-14
11:15
GBOT, Gaia and asteroids
Martin Altmann (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The Gaia-GBOT (Gaia Ground Based Optical Tracking) team is in charge of the optical tracking of Gaia itself. Since Gaia flies near the Ecliptic plane, where most asteroids happen to be found, the GBOT group started to look for serendipitous findings of asteroids on the nightly observations, identifying more than 5,000 objects in 12 months, about 40% of which are unknown. The already known objects are potentially even more interesting than the ''discoveries'' since GBOT looks at the opposition region, i.e. where main belt asteroids are generally closest and fully illuminated, which makes the photometry of these objects very important. I will give an overview of both programmes, the GaiaFUN-SSO (Gaia Follow Up Network-Solar System Objects) programme, and our own GBOT asteroid effort, and show how these two approaches can actually complement each other in an almost ideal way. Gaia will thus not only revolutionise our understanding of the Milky Way, but also, with a little help from GBOT, advance our understanding of small solar system bodies, which are indicators of the initial conditions of the formation of the Solar System.

2015-12-17
11:15
Als Astronom in Berlin und Heidelberg, und das je zweimal (As an astronomer in Berlin and Heidelberg, and this twice in both cities)
Roland Wielen (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Erinnerungen eines Emeritus. Überwiegend auf Deutsch, teilweise auf Englisch. (Reminiscences of an Emeritus. Mainly in German, partly in English.)

2015-12-10
11:15
The ZAH in the 'Virtual Observatory' (VO)
Markus Demleitner (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Over the past couple of years, many of your colleagues at the three ZAH institutes have published data to the VO (Virtual Observatory), quite a few of them at GAVO's Heidelberg data center. In this talk, I will present some of the services resulting from their contributions and show how they can be discovered and used as part of the larger Virtual Observatory framework - partly to show appreciation for our contributors, partly as an invitation of others to follow suit.

2015-12-03
11:15
The Physics of Cloud-Scale Star Formation in Galaxies across Cosmic Time
Diederik Kruijssen (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
A wide range of recent work shows that galactic star formation (SF) relations between the gas mass (density) and the star formation rate (density) develop substantial scatter or even change form when considered below a certain spatial scale. In this talk, I will show how this multi-scale behaviour of galactic SF relations can be exploited to determine ill-constrained, cloud-scale quantities such as the cloud lifetime, SF timescales, SF efficiencies, gravitational instability lengths, feedback timescales, and feedback efficiencies, using galaxy-scale observations. The method is validated using high-resolution numerical simulations of SF in disc galaxies. I will then present the first results of applying the formalism to observations, providing statistically representative measurements of the molecular cloud lifetime in M33 and M31, as well as using high-resolution ALMA Cycle 2 observations of the nearby flocculent spiral NGC 300. These lifetimes are used to put direct constraints on the star formation efficiency in molecular clouds and the emergence of the galaxy-scale SF relation. In the ALMA era, our new technique will enable the detailed characterisation of the SF process on the cloud scale in galaxies out to z ~ 4, i.e. across a cosmologically representative part of the galaxy population rather than the limited sample of Local Group galaxies where these measurements were previously possible. This enables the systematic study of SF and feedback physics as a function of the cosmic environment.

2015-11-26
11:15
Young galaxies at the highest redshifts
Leopoldo Infante (Pontificia Universidad Catolica, Santiago)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
I describe the progress made in the last few years regarding young galaxies in the high redshift universe. I focus on searches carried out using the Hubble Space Telescope CLASH and Frontier Fields galaxy cluster surveys. I describe how gravitational lensing by clusters increases the chance of finding z=7-10 galaxies considerably. By fitting spectral energy distributions, we provide insights into the star-formation rate, stellar masses and ages of the first galaxies at the end of “cosmic dark ages”. I present the faintest and one of the farthest (redshift z=10) galaxies ever observed. It existed only 500 million years after the Big Bang. We named this galaxy Tayna, which is the word for “first-born” in Aymara.

2015-11-25
11:15
Stellar Variability in the VVV Survey: An Update
Marcio Catelan (Pontificia Universidad Catolica, Santiago)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The Vista Variables in the Vía Láctea (VVV) ESO Public Survey consists of a near-IR variability survey of the Milky Way bulge and an adjacent section of the disk, using ESO's 4.1m VISTA telescope. It comprised 1929 hours of observations, over a timespan of more than five years, covering 109 point sources within a sky area of 520 square degrees, which includes 33 known globular clusters and ~350 open clusters. Here I provide an overview of the project's current status and future prospects, as well as some science highlights, with particular emphasis on recent results obtained using selected types of variable stars in the studied fields.

2015-11-19
11:15
Stellar Tidal Streams beyond the Local Group
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Within the hierarchical framework for galaxy formation, merging and tidal interactions are expected to shape large galaxies up to the present day. While major mergers are quite rare at present, minor mergers and satellite disruptions - that result in stellar streams - should be common, and are indeed seen in the halos of the Milky Way and the Andromeda galaxy. In the last years, the Stellar Tidal Stream Survey (PI. Martinez-Delgado) has obtained ultra-deep, wide-field imaging of some nearby spiral galaxies, based on data taken with a network of small robotic telescopes (0.1-0.5-meter). These images have revealed for the first time external views of such stellar tidal streams at unprecedented sensitivity and detail. In this talk, I present the new results of our systematic survey of streams in the halos of nearby Milky Way-like galaxies with the ultimate aim of estimating the frequency, morphology and stellar luminosity/mass distribution of these structures in the local universe. I also discuss recent follow-up observations (e.g. Spitzer, Keck, Subaru) and N-body modeling of the most striking streams and what we can learn from the comparison of the results of our survey with L-CDM cosmological simulations of stellar halos.

2015-11-12
11:15
Disentangling dark and luminous matter in galaxies through chemo-dynamical modeling
Glenn van de Ven (MPIA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Tests of the concordance cold dark matter model on the scale of galaxies are so far inconclusive due to our poor understanding of the interplay between baryons and dark matter (DM). Through the construction of dynamical models that fit the observed motions of stars we can infer the total mass distribution. Stellar population models that fit the observed chemical properties of stars provide the stellar mass-to-light ratio needed to convert the observed light of stars into a stellar mass distribution. Subtracting the latter stellar (and, if present, gas) from the total mass distribution yields the distribution of dark matter. In this presentation, I show how the combination of both dynamical and stellar population models allows us to disentangle the luminous and dark matter in galaxies of all types and of all masses from dwarf spheroidal to giant elliptical galaxies. At the same time, our chemo-dynamical modeling approach also provides insights into the formation history of galaxies, from a kinematically detected stellar stream as witness of a merger between dwarf galaxies, to the dynamical decomposition of giant galaxies into bulges and counter-rotating thin and thick disks.

2015-11-05
11:15
Adventures of a Gaia First Look Scientist
Michael Biermann (ARI / LSW)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Since 22 months, the Gaia First Look Scientists and Payload Experts Group are almost daily looking into the Gaia science and housekeeping data in order to check Gaia's instrument health and to judge the scientific data quality. In more than 100 weekly reports, over 100 problems and oddities were/are reported and discussed, out of which 85 have been understood and taken care of already. We present an overview of this work with some exciting examples, including news about Gaia's present status.

2015-10-29
11:15
Measuring the kinematics of Galactic globular clusters with VLT/X-shooter integrated-light spectroscopy
Frederik Schoenebeck (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
In this presentation I will report on the final results of my recently finished PhD project, in which I reduced and analyzed integrated-light spectra of eleven Galactic globular clusters (GCs) taken with VLT/X-shooter. Our observations were performed in an exceptional drift-scan mode, in which the spectrograph slit is slewed across the cluster at different radii. After presenting my (rather unconventional) data set, I will explain our methodology to construct accurate synthetic spectra from available resolved Hubble Space Telescope photometry, which we subsequently used as spectral templates in the determination of the GC kinematics via spectrum fitting. The obtained radial velocity profiles indicate ordered rotation for some clusters, while the velocity dispersion profiles of some GCs show potential imprints of intermediate-mass black holes residing in the GC cores. The integrated-light nature of our data, however, prevents an unambiguous interpretation of the obtained results and necessitates customized follow-up numerical simulations of the cluster dynamics. In the final part of my presentation I will report on the obtained dynamical cluster mass estimates, which support our general idea of a cluster's dynamical evolution being driven by evaporation of stars and gravitational interaction with the Galactic potential.

2015-10-22
11:15
Linking the Galaxy's Star Formation Process to Molecular Cloud Properties
Andreas Schruba (MPE Garching)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Understanding how galaxies transform their gas reservoir into stars is critical to understand galaxy evolution. I will present our state-of-the-art observational picture on the relationship between gas and star formation rate (SFR) surface density from large-scale mapping of HI, CO, HCN, and SFR tracers across nearby disk galaxies. We find that the SFR is regulated by two steps: the formation of a dense gas phase and the efficiency of star formation in the dense gas. Both steps sensitively depend on the galaxy's large-scale properties as well as on its molecular cloud properties. Subsequently, I will present a new consensus on the shared properties of molecular clouds in disk galaxies but also present environments where these systematically deviate. Finally, I will highlight current efforts to determine time scales of the processes studied above which are the last missing ingredient to a complete model of galaxy evolution.

2015-10-15
11:15
Morphological evolution of the cluster red sequence
Roberto de Propris (Tuorla Obs., Turku (Finland))
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Galaxies arrive on the red sequences of clusters at high redshift (z>1) once their star formation is quenched and evolve passively thereafter. However, cluster red sequence galaxies (CRSGs) undergo significant morphological evolution after the cessation of star formation. We find that over the last 5-6 Gyr there is little or no morphological evolution in the population as a whole, unlike in the previous 4-5 Gyr. During this earlier time (i) disk-like systems progressively disappear while (ii) the range of their axial ratios similarly decreases, removing thin disk systems, and (iii) radial color gradients (bluer outwards) decrease from age-related to metallicity-induced gradients. The effective radii suggest much less growth than for comparable field galaxies. A simple model of fading and thickening of a disk component after recent quenching around an otherwise passively evolving older spheroid seems to be consistent with all of these findings.

2015-07-24
11:15
The HD-PUC Graduate Student Exchange Program
Thomas Puzia (Pontificia Universidad Catolica de Chile, Santiago)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The Institute of Astrophysics at the Pontificia Universidad Catolica in Santiago, Chile and the Zentrum für Astronomie der Universität Heidelberg maintain a joint Ph.D. degree program in astronomy based on the existing Ph.D. programs at both universities. This double-degree program is intended to develop a lively exchange between the two universities, to offer seminars, summer schools, symposia, and workshops at both universities, to foster and develop new joint research projects and stimulate new inter-department programs. Graduate students from each university may apply for DAAD-funded stipends to spend time working at the partner university. During that time the students are members of the research staff of their host university with all corresponding rights. I will briefly highlight the ongoing research activities at the Institute of Astrophysics in Santiago and present the options for graduate students to spend time in Chile.

2015-07-16
11:15
Searching for globular-cluster escapee stars in the Galactic halo
Maria Cordero (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Stars born in globular clusters can be lost to the Galactic halo as a consequence of internal dynamical evolution or tidal interactions. Using SDSS low-resolution spectra we have identified a subsample of halo giants with strong CN absorption and weak CH absorption. These stars have the chemical composition typically found in second-generation globular-cluster stars, which shows that globular clusters contribute to the Galactic halo assembly. I will present an estimate of the fraction of halo field stars originally formed in globular clusters.

2015-07-09
11:15
Cosmic magnetism in simulations of galaxy formation
Volker Springel (HITS / ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Recent years have seen impressive progress towards hydrodynamic cosmological simulations of galaxy formation that try to account for much of the relevant physics in a realistic fashion. I will briefly review the state of the field in this area, highlighting a number of recent results obtained in galaxy formation with different simulation techniques. Specifically, I will discuss simulations that aim to produce disk galaxies similar to the Milky Way using a consistent embedding in the cosmological framework of LCDM, and the question of the origin of the magnetic fields in galaxies.

2015-07-02
11:15
Cosmological satellites and heating of Milky Way disk
Reza Moetazedian (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The presence of baryons in galaxies similar to the Milky Way has an important impact on both the host and the surrounding DM substructures. I have extracted DM properties from the Aquarius and Via Lactea simulations for Milky-Way-like host halos and their substruc-tures in order to realize them in the presence of baryonic matter in the form of disk and bulge of the host halo. The main goal has been to investigate the impact of subhalos on the dynamics of the MW disk and to answer the question if the current models allow us to re-produce the observed dynamics of the Galactic disk. These studies are carried out using an N-body particle-mesh code including the bulge, disk, main halo and the satellites as live components. This study is first of its kind to address the question of the observed velocity dispersion and heating of the MW disk in the presence of mergers with satellite galaxies using realistic initial conditions (ICs) extracted from cosmological simulations. The compari-son of the results from different cosmological ICs provides a measure of the required local DM population.

2015-06-25
11:15
Weak gravitational lensing and intrinsic alignments of galaxies
Björn Malte Schäfer (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
Weak lensing is one of the primary probes to investigate cosmic structure formation and to measure cosmological parameters. It operates under the assumption of intrinsically uncor-related galaxy shapes, which is flawed due to the interactions of galaxies with the surround-ing large-scale tidal fields in a number of different processes. After a review of the most important alignment processes I discuss the interference between lensing and intrinsic shape correlations, and forecasts on the estimation of cosmological parameters, and report on a couple of results from our group concerning intrinsic alignments in tomographic weak lensing surveys, in 3d weak lensing, and in higher-order correlation functions, and on future developments concerning the theory of tidal interaction.

2015-06-18
11:15
AGB Carbon Stars as Probes of the Milky Way - Investigating Sagittarius, and Possible Evidence for an Extended Disrupted Galactic Disc
Avon Huxor (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
I have been assembling a robust sample of AGB carbon stars in the Milky Way 'halo', whose variability permits a good estimate for distance. I will briefly summarise the sample, and earlier results on the location of the accreting Sagittarius dSph. I shall also discuss, in detail, a small group of carbon stars, lying in Triangulum-Andromeda. The presence of long-period variables in this group suggests a relatively young age, more consistent with the Galactic Disc than the tidal tails of an accreting satellite. These stars provide additional evidence for the recent proposal for an extended, disrupted disc upto a Galactocentric distance of 25 kpc, and many kpc from the Galactic Plane.

2015-06-11
11:15
Habilitations-Vorstellungsvortrag : Probing the Microscopic with the Macroscopic: from Properties of Star Cluster Systems to Properties of Cluster-Forming Regions
Genevieve Parmentier (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
In this presentation, I will show how the properties of star cluster systems (i.e. the macroscopic scale) can be exploited to probe into the formation conditions of star clusters (i.e. the microscopic scale). Specifically, I will build on the evolution with time of the cluster mass distribution to constrain the mass-radius relation of clusters at birth, and to explain the limit for massive star formation observed in the mass-radius space of molecular clouds. These results have stimulated the development of a new model for cluster formation whose applications and consequences will be discussed: (i) the star formation relation of the Solar neighbourhood, (ii) the improved survivability of clusters after gas expulsion, and (iii) stellar age spreads in clusters as a function of their mean volume density.

2015-05-28
11:15
How environment shapes galaxy evolution: the satellite galaxies' perspective
Anna Pasquali (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
It is by now well established that galaxy evolution is driven by intrinsic and environmental processes, both contributing to shape the observed properties of galaxies. A number of early studies, both observational and theoretical, have shown that the star formation activity of galaxies depends on their environmental local density and also on galaxy hierarchy, i.e. centrals vs. satellites. Contrary to the central (most massive) galaxy of a group/cluster, satellite galaxies are stripped of their gas and stars, and have their star formation quenched by their environment. Large galaxy surveys like SDSS now permit us to investigate in detail environment-driven transformation processes by comparing centrals and satellites. In this talk I will discuss what we have so far learned about environmental effects by analyzing the observed properties of local central and satellite galaxies in SDSS, as a function of their stellar mass and the dark matter mass of their host group/cluster.

2015-05-21
11:15
Galactic Star Cluster mass evolution. High performance star by star simulations including the central Intermediate Mass Black Hole (IMBH)
Peter Berczik (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
We carry out the large scale Galactic Star Cluster (SC) simulations (N = 1M & M = 5.74E+05 Msol initial mass) using our high performance parallel direct N-body code phi-GPU with the maximum possible numerical resolution (one particle one star) on the largest astrophysical GPU clusters in Germany and China. Our main goal was to investigate the possible influence of the central IMBH on the radial stellar mass distribution in different mass ranges including remnants, especially near the star cluster center. The current observational evidence for the existence of IMBH objects in Galactic star clusters is still quite inconclusive. This is the main reason why we need a better and more detailed theoretical dynamical study of such exotic Galactic objects.

2015-05-07
11:15
Galaxy formation in a dark universe: the NIHAO project
Andrea Macciò (MPIA)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
In a Universe dominated by Dark Matter and Dark Energy, galaxy formation is a complex mixture of hierarchical halo assembly, dissipational processes and secular evolution. I will start my talk with a brief summary of the techniques used for performing cosmological numerical simulations, from pure gravitational simulations to hydrodynamical ones. I will then review the past years efforts of my research group at the MPIA in creating realistic (disk) galaxies in a cosmological context, with particular emphasis on the role of stellar feedback and cosmological background. I will then introduce the NIHAO project, a new, unmatched, suite of high resolution cosmological simulations. I will highlight the current and future projects based on these simulations, especially regarding the effects of baryons on dark matter distributions in galaxies. I will then conclude my presentation by talking about the current challenges in reproducing the properties of massive, elliptical galaxies.

2015-04-30
11:15
Calendar basics
Robert Schmidt (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
There is a long tradition of calculating calendar data at ARI. The annual volume "Astronomische Grundlagen für den Kalender", edited by ARI, provides calendar publishers in Germany with basic material for the production of calendars. In this talk we sum up some of the joys and pitfalls encountered in our first two years of continuing this effort. We exemplify the interest of the general public in these data and address well-known and less well-known feasts, as well as their mechanical representation.

2015-04-23
11:15
A microlensing search for exoplanets on a robotic network of telescopes
Yiannis Tsapras (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
The Las Cumbres Observatory Global Telescope Network (LCOGT) is a young organization dedicated to time-domain astronomy. It operates a homogeneous network of robotic telescopes on multiple sites around the world. The telescopes are controlled by software capable of orchestrating complex and highly responsive observing programs using the entire network to provide round-the-clock monitoring of any astronomical target of interest. The network delivers high cadence observations of ongoing Galactic microlensing events facilitating the detection of brief planetary anomalies. These discoveries have revealed a population of exoplanets in wide orbits around low-mass stars which lies beyond the sensitivity limits of conventional transit and radial velocity surveys. Several of these exoplanets have masses that lie in the range 1-18 Earth masses, providing compelling independent evidence that low mass planetary bodies are far more common than gas giants around stars in the Galaxy. Among these discoveries we also find a number of massive Super-Jupiters orbiting their dwarf host stars well beyond the snow-line. This particular class of objects poses a problem for current theories of planetary system formation.

2015-04-16
11:15
First glimpse into Gaia's astrometry - a progress report
Ulrich Bastian (ARI)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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Abstract
According to the official plan, the first astrometric data releases from Gaia are expected by mid 2016 (positions only) and mid 2017 (incl. parallaxes and proper motions). This is the time needed to collect enough data to make a global astrometric sphere solution mathematically possible, and then to understand and calibrate these data precisely and reliably enough to get meaningful scientific results from them. Both processes have been in full swing since mid 2014. We have collected eight months of beautiful data, and we are getting more and more insight into the intricacies of the data, on the one hand - and into their great potential, on the other hand. I will show and explain some of the problems and initial failures encountered so far, and some of the successes achieved meanwhile.

2015-03-09
11:15
The Impact of World War I on Chemistry, Astronomy, and Physics
Virginia Trimble (University of California, Irvine)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The second world war (WWII) is generally called the physicists' war (radar, rockets, nuclear weapons) and the first world war (WWI) the chemists' war (poison gases, but also synthetic rubber, petroleum substitutes, and dyes). Both wars had an enormous impact on all of science. For us astronomers, WWI began with a German eclipse expedition to the Crimea, led by Erwin Freundlich, being taken prisoners and interned. They had gone primarily to attempt to measure the bending of starlight by the sun, which a 1911 Einstein paper said should be 0.81". The war ended with the Eddington solar eclipse expeditions, which really did see the bending in 1919. That year also saw the founding of the International Astronomical Union and other scientific organizations under the rules set by the Treaty of Versailles. In between came many deaths, Karl Schwarzschild probably being one of the best known. Observatory optical shops were turned over to makers of prisms for periscopes and binoculars. Many scientists served on both sides on multiple fronts, and the gentleman who refereed the second of the papers from my PhD dissertation had served the full 4-plus years, mostly on the Eastern Front (and there will be a small prize at the end of the talk for the first person to identify him).

2015-02-05
11:15
A search for tidal features around low-mass stellar systems in nearby galaxy clusters
Carolin Wittmann (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
Based on deep wide-field-imaging data, we search for tidal features around the most diffuse and the most compact low-mass galaxies in the centers of the Perseus and the Fornax galaxy clusters. In Perseus we investigate faint dwarf galaxies to probe whether they are currently getting disrupted by the cluster potential or being protected by large amounts of dark matter. In Fornax we examine UCDs for signs of being remnants of disrupted dwarf galaxies. Within the dwarf galaxy population of the Perseus cluster, we find one galaxy with tidal tails and others with possible signs for tidal perturbations. In the Fornax cluster, we do not find diffuse tidal debris around UCDs, but some of them have large radii. We interpret our results in the light of the timescales and frequency of disruption events as predicted from simulations.

2015-01-29
11:15
Imprints of galaxy evolution on HII regions
Anna Pasquali (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
HII regions are the signpost of the most recent episodes of star formation in a galaxy. Their gas, ionized by their young and massive stars, is commonly used to measure the present-day star-formation rate and gas-phase metallicity of their host galaxy. By means of IFU spectroscopy of about 300 nearby galaxies, the CALIFA survey has identified more than 5000 HII regions across a wide range of galaxy Hubble type, stellar mass, age, and metallicity. We have studied the distribution of these regions on the classical Baldwin, Philips & Terlevich's (1981) diagram as a function of their properties first (i.e. ionization parameter, electron density and oxygen abundance), then by the properties of their host galaxies. The results indicate that HII regions "keep memory" of the star-formation history and chemical enrichment undergone by their host galaxy.

2015-01-22
11:15
Visualisation of Gaia for public outreach
Toni Sagrista (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The Gaia Sandbox is an open source, real-time, 3D, astronomy visualisation software developed in the framework of ESA's Gaia mission outreach. Its aim is to offer a powerful and yet easy to use tool that runs everywhere (Linux, MacOS, Windows, Android and eventually iOS and WebGL) to visualise several aspects related to astronomy in general and Gaia in particular and eventually its catalogue. It contains a fully- featured recreation of our Solar System and the Galaxy and uses advanced graphics techniques to render beautiful, appealing scenes. In this talk I will present this piece of software and explain how one goes about building such a tool. I will also present our future plans and finalize the talk with a live demonstration.

2015-01-15
11:15
The local stellar luminosity function in the NIR
Andreas Just (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The characterization of stellar populations in the near infrared (NIR) is gaining increasing importance due to the growing number of Galactic and extragalactic surveys in the NIR. A crucial parameter is the mass-to-light ratio M/L needed to convert the observed luminosity to the underlying mass distribution. Traditionally, optical bands are used to determine the mass of the stellar population. But in the B- and V-bands the total luminosity is dominated by young and massive stars carrying only a low mass fraction. This leads to a bias in the mass determination of mixed populations such as in the discs of spiral galaxies. The determination of the disc mass, needed for example to disentangle the baryonic and dark matter contribution to the rotation curve, is typically uncertain up to a factor of 2. It has long been known that M/L in NIR bands is much less sensitive to different star formation histories. The solar neighbourhood is still the only place where a direct calibration of M/L for disc populations is possible. Based on Hipparcos stars and the Catalogue of Nearby Stars combined with 2MASS data we have determined the luminosity function in the NIR and derived the M/L in the K-band. Method and properties will be discussed in detail.

2015-01-08
11:15
Velocity dispersion measurements of globular clusters with X-shooter spectroscopy
Frederik Schönebeck (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1

2014-12-18
11:15
Streams, stars, and dwarfs in the Milky Way and M31 halos
Andreas Koch (LSW)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The question of how galaxies like our Milky Way (MW) formed and evolved in a hierarchical framework remains a key area of contemporary astrophysics. In this talk I will address this question by focusing on the halos of the MW and the near-by Andromeda galaxy and their inhabitants. Thus intercomparing star clusters, dwarf galaxies, field stars, and tidal streams we can not only study the formation histories of the halos, but also investigate the internal evolutionary histories of their building blocks on their own merit. Here I will show how we can use chemical element abundances to identify objects as either dark-matter dominated dwarf galaxies vs. mere extended star clusters or tidal overdensities. Furthermore I will introduce a new survey that explicitly targets the intermediate-age population of the M31 halo so as to trace more recent merger events. Finally I will discuss the hunt for gas in these systems, as seen through absorption in distant quasar spectra.

2014-12-11
11:15
AGN jets as cosmological standard rulers
Jonas Frings (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The geometry of the Universe is mapped out with two primary techniques: supernovae as standard candles and fluctuations in the cosmic microwave background (and later the galaxy distribution) as standard rulers. In my master thesis I am investigating whether the angular velocity of jets in active galactic nuclei can be used as a standard ruler: In fact, motion in AGN jets can be measured over cosmological distance if jets are universal in the sense that their Lorentz-factors follow a known distribution, for which is some amount of observational evidence. I outline my statistical technique and present (preliminary) constraints on cosmological models.

2014-12-04
11:15
New low-mass members of the Octans moving group and an updated lithium age
Simon Murphy (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The Octans association is one of several young stellar moving groups recently discovered in the Solar neighbourhood and a valuable laboratory for studies of stellar and planetary evolution. We have undertaken the first spectroscopic survey for K and M-type Octans members, yielding 29 UV-bright stars with kinematics, photometry and distances consistent with existing members. Several stars have strong Li I absorption, from which we estimate a lithium age of 30-40 Myr, similar to that of the Tucana-Horologium association. The results of our survey show that Octans may be the dispersing remnant of a sparse, extended structure which includes some younger members of the recently proposed foreground Octans-Near association.

2014-11-27
11:15
Global Modes in the Milky Way: Bar Formation
Evgeny V. Polyachenko (INASAN (Inst. Astron. Moskau))
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
In the secular scenario of formation and evolution of spiral galaxies, gravitational instabilites play a major role. In this framework, we aim to reproduce the bar and the spiral structure of the Milky Way using linear stability theory and numerical simulations. As a first step, we test available methods for the determination of unstable modes in stellar discs. Using core and cusp models of galaxies, the existence of two different kinds of spectra of unstable modes are demonstrated. The strong and weak points of the methods are discussed. Numerical simulations of the Kuijken-Dubinski (1995) core model of the Milky Way reproduce a bar with radius of about 3 kpc, which is not destroyed for at least 4 Gyr. The bar pattern speed is 75 km/s/kpc in the beginning of the simulations, and about or less than 40 km/s/kpc at the end of the simulations, while the observed value is 59 ± 5 ± 10 (sys) km/s/kpc.

2014-11-20
11:15
Globular clusters in the Local Group
Eva Grebel (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
All of the more massive galaxies in the Local Group typically contain one or several globular clusters. A considerable fraction of the outer halo globular cluster population of the Milky Way and M31 may have been accreted from since disrupted satellites, whereas disrupted globular clusters contribute to the field star populations. While we tend to take the globular cluster system of the Milky Way as a template for disk galaxies in general, in recent years a number of intriguing differences in the kinematics, chemistry, and ages of such systems have emerged. Moreover, some recently discovered globular clusters seem to begin to fill the gap between clusters and dwarf galaxies.

2014-11-13
11:15
Redshift Space Distortions in the Galaxy Distribution
Alex Szalay (Johns Hopkins University, Baltimore, USA)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The talk will discuss how redshift space distortions due to the motion of galaxies affect a feature in the correlation function due to Baryon Acoustic Oscillations. The talk will also present results from analyzing the SDSS Main Galaxy Survey. We will discuss the impact of using unusual geometries in computing the galaxy correlation function. We show that redshift-space distortions can substantially sharpen the BAO peak in directions close to the line of sight. We also demonstrate a detection of Baryon Acoustic Oscillations even in this relatively shallow galaxy sample.

2014-11-06
11:15
LAMOST and its survey
Gang Zhao (National Astronomical Observatories of CAS, China)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
LAMOST is an impressive and highly innovative astronomical facility, unique worldwide. It comprises a 4 meter Schmidt telescope with a large active segmented reflecting corrector and a focal plane with a robotic 4000 fiber positioning system, which feeds 16 spectrographs capable of a range of dispersions. The combination of its wide field and large aperture offers unique prospects for major breakthroughs in studies of Galactic structure and extragalactic astronomy. After a commissioning phase, LAMOST started its one year pilot survey in September, 2011 and obtained 0.5 million spectra. Now it is operating in regular survey mode. Up to now, we released 2.2 million spectra with R = 1,800. In this talk, some recent LAMOST results are presented.

2014-10-30
11:15
Strong but not quite: gravitational flexion in galaxy clusters
Matteo Maturi (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
Gravitational lensing is very successful in recovering information both on large scales (weak lensing) and on small scales (strong lensing) but many difficulties still have to be faced in recovering the intermediate regime, i.e. gravitational flexion. Being very sensitive to substructures, flexion would be of great help in completing our understanding of galaxy clusters. Although it is based on very clean physics, its actual measure is complicated by intervening spurious contributions and observational effects. In this talk I am going to discuss the main aspects that have to be confronted to recover this challenging and powerful signal.

2014-10-23
11:15
The multi-scale nature of galactic star formation
Diederik Kruijssen (MPA Garching)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The unknown physics of star formation and feedback represent the main bottleneck in connecting the observable galaxy population to cold dark matter cosmology. Both physical processes are expected to vary strongly with the galactic environment and across cosmic history, but can only be observed in detail within the limited range of environments contained by the Local Group. I will discuss the three main avenues we have recently developed to constrain the cloud-scale physics of star formation and feedback from nearby galaxies out to high redshift. Firstly, the Central Molecular Zone of the Milky Way is almost indistinguishable from high-redshift star-forming galaxies in terms of its gas properties, but due to its close distance it can be observed and modelled in great detail. Secondly, the globular clusters hosted by massive galaxies in the local Universe bear the imprints of the conditions under which they formed, which can be uncovered by modelling the co-evolution of globular clusters and their host galaxies. Thirdly, we have recently developed a new method applicable out to z > 4 to directly measure cloud-scale star formation time-scales and efficiencies, as well as the feedback energy and momentum deposition rates, without requiring individual clouds to be resolved.

2014-10-16
11:15
Local-Density-Driven Clustered Star Formation: Model and (Some) Implications
Genevieve Parmentier (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
A super-linear power-law trend between the local surface densities of molecular gas and young stellar objects in molecular clouds of the Solar Neighbourhood has been identified by Gutermuth et al. I will show that models of cluster formation in which the gas is turned into stars with a constant star formation efficiency per free-fall time easily reproduce such a steep local star formation relation. Aspects such as (i) the survivability of clusters after they expel their residual star-forming gas, (ii) the inferred star formation efficiency per free-fall time, and (iii) stellar age spreads in clusters as a function of their mean volume density will also be discussed.

2014-07-24
11:15
(to be announced)
NN (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1

2014-07-17
11:15
Panchromatic High Resolution Spectroscopy of Local Group Star Clusters
Frederik Schönebeck (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1

2014-07-10
11:15
Stäckel potential approximation and local constraints in the extended solar neighbourhood
Alberto Nardin (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The problem of the "third integral of motion" has been studied for decades. Its existence was determined at the beginning of numerical experiments with orbits in an axisymmetric potential and by observations of triaxiality in the velocity dispersion of nearby stars, but in general its exact expression is not possible to derive analytically. The only way to find an exact third integral of motion is to approximate the gravitational potential with a potential of the Stäckel form. It is linked with the phase-space distribution function and the velocity ellipsoid; therefore it is an important ingredient for the construction of a dynamical model of the extended solar neighbourhood. I will present such a model, which reproduces the observables in this region of interest.

2014-07-03
11:15
Simulating the stellar initial mass function in a turbulent environment
Clio Bertelli Motta (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The (non-)universality of the stellar initial mass function (IMF) has been the subject of an intense debate in the last years, especially due to spectroscopic and dynamical data that seem to suggest that the IMF of early-type elliptical galaxies differs from a standard Chabrier or Kroupa IMF and varies with galaxy mass. From a theoretical point of view, this issue translates to a question about the physical processes influencing the shape of the mass distribution of stars at their birth. It is commonly accepted that supersonic turbulence plays a major role in the process of gas fragmentation and consequently of star formation and several analytic theories have studied its influence on the IMF. Our study is based on smoothed-particle hydrodynamics (SPH) simulations of a star forming region characterised by a periodically driven turbulent velocity field. In particular, we investigate the influence of variations in the turbulent Mach number of the gas on the resulting IMF. We find that for an increasing velocity dispersion, the mass distribution of the protostellar objects becomes progressively top-heavy, contrarily to the predictions from analytic theories. We argue that this discrepancy emerges from poor mapping between the “core“ mass function and the initial mass function, due to the turbulent disruption of a significant fraction of the dense cores.

2014-06-26
11:15
Constraining the initial mass function by modelling the distribution of nearby stars
Jan Rybizki (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The initial mass function has seen numerous re-determinations since the seminal work of Edwin Salpeter in 1955. Originally the IMF was derived by luminosity functions via the luminosity-mass relation and stellar lifetimes. A recent approach to get hold of the theoretical concept of the IMF is to use galaxy models reproducing observations. I will present such an investigation using the Just and Jahreiß 2010 Milky Way disc model and Galaxia, a tool to generate mock observations. The data that are used come from the in-house Catalogue of Nearby Stars and from Hipparcos. An important aspect in this approach is the goodness-of-fit between model and observations and I will highlight an improved statistical measure for 2-dimensional data sets.

2014-06-12
11:15
All-Sky Automated Survey for Supernovae: Big Science with Small Telescopes
Krzysztof Stanek (OSU)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
Even in the modern era, only human eyes survey the entire optical sky for the violent, variable, and transient events that shape our universe. To change this, we have built and implemented the All-Sky Automa-ted Survey for Supernovae (ASAS-SN). This is a long-term project designed to monitor the extragalactic sky down to V~17 mag every 2-3 days using multiple telescopes, hosted by Las Cumbres Observatory Global Telescope Network, in the northern and southern hemispheres. Our telescopes consist of commercially available telephoto lenses and CCDs, so future expansion is straightforward. The primary focus of the survey is to find bright nearby supernovae (SNe) and other transient sources. We began running our real-time search for variable sources in late April 2013 with our first unit, "Brutus", and in May 2014 we have deployed "Cassius" in Chile. ASAS-SN has already found 20+ bright nearby SNe and outbursts from 100+ cataclysmic variable stars, many M-dwarfs, young stellar objects (YSO), AGN and a tidal disruption event 200 Mpc away. ASAS-SN is an ongoing survey which, judging by its current success and future expansion, promises to be innovative and prolific for years to come.

2014-06-05
11:15
Masers, 2MASS, Simbad: Bringing it all together
Markus Demleitner (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
Do you think the Virtual Observatory (VO) is just for nerds? This talk’s goal is to convince you of the opposite: The VO is for you! We will show simple examples and provide you with recipes for standard tasks such as finding relevant astronomical data, crossmatching them with remote resources, fast visualisation, selecting by physical properties, filtering items by Simbad classifications, and follow-up queries for further data. If you are doing any of this more often than once a year (and do not already use the VO), the hour spent on this talk will almost certainly be well invested.

2014-05-22
11:15
Weighing the Milky Way with High Precision Using Tidal Streams of Globular Clusters
Andreas Küpper (Columbia University, New York)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
Dissolving globular clusters produce cold tidal streams in the Milky Way halo which can span up to several degrees on the sky. These streams trace the gravitational field of the Galaxy’s dark halo and enable us to measure its mass and shape. On the example of the most prominent globular cluster stream, Palomar 5, I will demonstrate how existing data from imaging surveys like SDSS and from spectroscopic campaigns can be used to constrain models of the stream progenitors. In particular, I will show how dynamical sub-structure in the stream helps to differentiate between models. From the modeling we can infer the shape of the Galactic dark halo as well as the mass, distance and orbit of the globular cluster. Future survey data from, e.g., Gaia and LSST will reveal large numbers of streams in the Milky Way and allow us to probe the Galactic potential with increasing precision.

2014-05-15
11:15
Mapping the environmental footprint of galaxy clusters
Thorsten Lisker (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The "environmental footprint" is commonly understood as our own impact on the environment. Here we reverse it and ask: how can we measure the impact an environment has on its residents, its galaxies? Can we identify signatures of the past and present environmental influence in the observed galaxy properties, and can that help us to understand how the nearby galaxy clusters have been assembled? I would like to present new observational results on the internal dynamics, structure, and stellar populations of low-mass galaxies in nearby galaxy clusters. By analyzing how these properties vary with location and environmental characteristics, we aim to reach our goal of using the abundant low-mass galaxies as messengers that tell of an environment's history and its impact on galaxy evolution.

2014-05-08
11:15
Dynamical Evolution of Planetary Systems in Star Clusters
Rainer Spurzem (National Astronomical Observatories, Chinese Academy of Sciences)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
The majority of stars form in star clusters and many are thought to have planetary companions. We show direct N-body  simulations and Monte Carlo experiments for single and multiplanet systems embedded in star clusters. The latter are more prone to instabilities as a result of frequent stellar encounters in these star clusters than single-planet systems. We find that in the equal-mass planet model, 70 per cent of the planets with initial semimajor axes a > 40 AU are either ejected or will collide with the central star or another planet within the lifetime of a typical cluster, and that more than 50 per cent of all planets with a < 10 AU remain bound to the system. The rate of creation of free floating planets is discussed. Planets with short orbital periods are not directly affected by encountering stars. However, secular evolution of perturbed systems may result in the ejection of the innermost planets or in physical collisions of the innermost planets with the host star, up to many thousands of years after a stellar encounter. Our results indicate that stellar encounters can account for the apparent scarcity of exoplanets in star clusters. They act not only through direct effects, but also through de-stabilization of multi-planetary systems, which can affect inner planets long after a stellar encounter has perturbed the planetary system.

2014-04-24
11:15
Gaia news and status
Marcus Hauser (ARI)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
I will report on the recent status of the ongoing Gaia commissioning campaign. The most important achievements and problems encountered since the last presentation in February will be highlighted.

2014-04-17
11:15
Unveiling 2D and 3D Images of Astrophysical Gas Flows in Interacting Binary Star Systems
Mercedes Richards (Penn State University)
ARI Institute Colloquium
ARI, Seminar Room 1
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Abstract
Interacting binary star systems are in the process of mass transfer during which gas will flow along a path called the gas stream, from the more evolved mass-losing star to its companion. Images of these gas flows have not yet been realized with direct-imaging techniques because such details cannot be resolved even with the largest telescopes. However, the image reconstruction technique of Doppler tomography has revealed 2D images of gas flowing along the predicted gravitational path between the stars, accretion disks or accretion annuli around the mass-gaining star, hot spots where the gas flow collides with the surface of the star, and evidence of magnetic flows connected with the cool magnetically-active mass-losing star. The extension to 3D tomography has unveiled views of extensive gas flows beyond the central plane of the binary system. These new 3D images show that the magnetic field of the mass losing star can influence the gas motions dramatically in some cases. Hence, tomography has revealed the active environments that exist in a range of objects from direct-impact binaries to cataclysmic variables and gamma ray binaries.

-0001-11-30
00:00
In-situ IMF at z~2
Themiya Nanayakkara (Swinburne University)
ARI Institute Colloquium
ARI Moenchhofstrasse 12-14, ARI Seminar Room 1
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Abstract
The development of sensitive Near Infra-Red instruments has made it possible to study galaxy properties at z~2, just 3Gyr after the Big Bang. This is expected to be the time period where galaxies are actively star forming and evolving rapidly to form the massive galaxies that are observed in our local neighbourhood. ZFIRE is a survey, which utilises the MOSFIRE instrument on Keck telescope over 18 nights to study properties of ZFOURGE selected mass complete galaxies in rich environments at z~2. In my talk I will present results of the first ever attempt to constrain the Initial Mass Function (IMF) of galaxies at these redshifts using a cluster and a field sample. We have investigated the degeneracy between the star formation histories and the IMF to make strong constrains on the stellar mass distribution of these galaxies using synthetic stellar spectra. I will focus on the role of dust, star-bursts, stellar rotation, binaries, and metallicity on determining observed galaxy properties at z~2 to address whether ZFIRE results favour the canonical concept of a universal IMF.