Events, Seminars, Talks

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

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The last five years have seen enormous progress in mapping local galaxies with integral field spectroscopy. Large surveys, like CALIFA, SAMI and MaNGA, have now provided us with a detailed view of star formation and chemical enrichment across the Hubble sequence at z=0. In this talk I will review what these observations have taught us about the chemo-dynamics of local disc galaxies, focusing on metallicity gradients as probes for disc assembly.

Moving beyond metallicity gradients requires mapping of ISM at the ?cloud-scale?, enabling us to resolve individual HII regions from the diffuse ionised gas background. I will present early results from the PHANGS-MUSE survey, a large program with the MUSE integral field spectrograph, which has recently obtained the first systematic view of the ionised ISM at the cloud scale (<\;50 pc) across a variety of local environments in a sample of 19 nearby galaxies. This dataset is allowing us to take the first step towards drawing a complete picture of the baryon cycle, and bridging the gap between the Milky Way and the extragalactic view of star formation and chemical enrichment.

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

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Gaia aims at producing micro-arcsecond-precision astrometry of a vast number of stars and other celestial sources. This involves the task of reconstructing and understanding the rotational motions of the free-floating Gaia spacecraft to micro-arcsecond-precision. The Gaia "First Look" group of ARI is in charge of the root steps of this task, plus a daily in-depth verification of the instrument health on board and the scientific quality of the data coming to the ground. Sounds boring? But it isn't! I will describe and explain a few of the adventures of Gaia's "First Look Scientists" at ARI, including a very recent one.

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

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

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

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High-redshift radio galaxies (HzRGs) are hosted by some of the most massive galaxies known at any redshift and are unique markers of concomitant powerful active galactic nuclei (AGN) activity and extreme starbursts. Their energetic radio jets, high star formation rates and black hole accretion rates place them amongst the most active sources at and near Cosmic Noon. Their extended gaseous environments of HzRGs are disturbed by outflows and inflows and show signs of significant jet-gas interactions making them unique objects in which quasar-mode feedback, radio-mode feedback and the host galaxies can be studied simultaneously. I will present Multi Unit Spectroscopic Explorer (MUSE) integral field unit spectroscopic observations of the 70 kpc x 30 kpc Lyman-alpha halo around a massive (10^11.8 M_sun) z = 4.5 radio galaxy. I will present our detailed spatially resolved spectral analysis of the complex Lyman-alpha profile in which we identify and measure the signatures (kinematics and column densities) of eight neutral gas absorbing systems at -3500 < v < 0 km/s. The strongest absorber at v ~0 km/s has a high covering fraction being detected across the extent of the Lyman-alpha halo, a significant column density gradient along the south to north direction and a velocity gradient along the radio jet axis. The absorber is also observed in in CIV and NV absorption, and very likely represents an outflowing metal-enriched shell driven by a previous AGN or star formation episode within the galaxy and is now caught up by the radio jet leading to jet-gas interactions. These observations provide evidence that feedback from AGN in some of the most massive galaxies the early Universe may take an important part in re-distributing material and metals in their environments. This work is part of larger sample of similarly massive, high-z radio galaxies and I will present the future plans for this unique sample of massive high-z galaxies hosting powerful AGN. JWST will be transformative for these galaxies as it will allow a detailed investigation on how radio- and quasar-mode feedback work together in the early Universe.

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The ROME/REA project (2017-2020) aimed to discover extrasolar planets by regularly monitoring millions of stars in the Galactic bulge and looking for ongoing microlensing events. From April to September each year, when the Galactic bulge was visible from the Southern hemisphere, the robotic telescopes of the Las Cumbres Observatory were used to observe a total area of about 4 square degrees in the sky in three different bands. An automated process assessed ongoing microlensing events in real time for their sensitivity to planetary signals and additional observations were requested to characterize signals of particular scientific interest. Our final catalog of stars contains more than 4 million individual sources. As we prepare for our first public data release, I will present some of the results, talk about the data products we will soon be releasing and describe our current work and plans for the future.

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

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

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