Ruprecht-Karls-Universität Heidelberg

ARI Institute Kolloquium


Upcoming events


2017-10-19
11:15
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Diederik Kruijssen's group (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2017-10-26
11:15
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Ulrich Bastian (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2017-11-02
11:15
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Christopher Usher (Liverpool John Moores University)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2017-11-09
11:15
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Diederik Kruijssen's group (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2017-11-16
11:15
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Maria Jimenez (ITA)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2017-11-23
11:15
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Linda Urich (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2017-11-30
11:15
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Eric Pellegrini (ITA)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2017-12-07
11:15
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Kseniia Sysoliatina (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2017-12-14
11:15
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Trifon Trifonov (MPIA)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2018-01-11
11:15
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Clio Bertelli Motta (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2018-01-18
11:15
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Carolin Wittmann (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2018-01-25
11:15
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Andreas Koch (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2018-02-01
11:15
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Fayezeh Shabani (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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2018-02-08
11:15
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Jasna Ordenes (PUC Chile,ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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Past events


2017-07-27
11:15
Dense stellar systems, supermassive black holes, tidal disruptions
Rainer Spurzem (ARI, NAOC, KJAA)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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
D. Martinez-Delgado (ARI)
ARI Institute Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12 -14, Seminar Room 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminarraum 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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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 Kolloquium
ARI Mönchhofstrasse 12 - 14, Seminar Room 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
ARI Moenchhofstrasse 12-14, Seminar Room 1
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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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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
David Martinez-Delgado (ARI)
ARI Institute Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
ARI, Seminar Room 1

2014-07-17
11:15
Panchromatic High Resolution Spectroscopy of Local Group Star Clusters
Frederik Schönebeck (ARI)
ARI Institute Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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 Kolloquium
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.

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