Vorträge, Seminare, Ereignisse

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

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Spatial modulations in the observed galaxy luminosity distribution (derived from redshifts as distance proxies) trace the radial peculiar velocity field. Using spectroscopic redshift surveys, this signal can be used to obtain independent bounds on large-scale flows and cosmic growth, complementary to traditional methods (such as clustering analysis). Considering SDSS galaxies at z=0.1, I will present recent measurements of the cosmic growth rate based on this approach and discuss ongoing developments that aim at extending the general methodology for other purposes such as applications to photometric redshift catalogs.

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The measurement of stellar properties such as chemical compositions, masses and ages, through stellar spectra, is a fundamental problem in astrophysics, of importance to many fields (e.g. Galactic archeology, chemical evolution, planetary formation, etc.). I discuss progress in the understanding of atomic collision processes relevant to high-accuracy non-LTE analysis of late-type stellar spectra. In particular, to analyse spectra at accuracies approaching the 1% level, quantum mechanical effects such as electron transfer by tunnelling and spin transfer via the exchange interaction should be accounted for in modelling collision processes involving electrons and hydrogen atoms. Similar processes are also important in the relatively young field of non-LTE analysis of spectra of supernova ejecta, showing even larger effects than in stellar atmospheres.

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I will question the notion of "completeness" for a star catalogue, and briefly present how it relates to the concept of "selection functions". I will illustrate the point with examples from Gaia data, in the hope of triggering a lively on-the-spot discussion among the audience.

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We study the formation of the bound clusters after instantaneous gas expulsion, when the cluster forms with centrally peaked star-formation efficiency (SFE) profile. That means our embedded cluster has steeper density profile than that of initial and residual star forming gas at the time of gas expulsion. This is the consequence of star-formation proceeding with a constant SFE per free-fall time in centrally-concentrated spherically-symmetric molecular gas clump according to the local-density-driven cluster formation model by Parmentier and Pfalzner (2013). We assume our embedded cluster in virial equilibrium with the gravitational potential of whole clump (star+gas) at the time of gas expulsion. We perform a grid of direct N-body simulations, starting from instantaneous gas expulsion time with different global SFEs and cluster initial stellar masses. We found that the minimum global SFE needed to survive instantaneous gas expulsion is 13 percent for the isolated cluster and 15 percent for the cluster in the Solar Neighbourhood.

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

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A prime objective of observational astrophysics is to characterize the earliest sources in the first Gyr of the universe, and to peer into the cosmic times when the first stars, black holes and galaxies formed. Although galaxy candidates are now identified up to redshifts of about 10, their faintness typically precludes detailed studies of their nature. Quasars, on the other hand, are the most luminous non-transient sources known and can be studied in detail at the earliest cosmic epochs. The discovery and characterization of a statistically significant sample of quasars at z>6 is crucial to study the epoch of reionization. I will present our progress in building such a statistical sample, which led to tripling the number of these quasars in just the last three years. I will discuss the diverse range of physical properties of this quasar sample as well as our follow-up studies from optical to radio wavelengths, including a new quasar at a record redshift (z=7.5). In particular, recent observations with ALMA revealed the presence of far-infrared companions around the quasars, and provide key constraints on the spatially resolved properties of the quasar host galaxies. Through multi-line ALMA spectroscopy we can also derive first constraints on the physical conditions of the interstellar medium in the quasar hosts. I will also discuss the potential of future JWST observations.

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I will discuss results from our recent work on the detailed chemistry of globular clusters (GCs) in Local Group galaxies. This work is based on a combination of integrated-light spectroscopy at high resolution and resolved imaging with HST in colour combinations that are sensitive to light-element abundance variations. Through the resolved imaging studies, it is now clear that the abundance anomalies that are the hallmark of multiple stellar populations are not restricted to old GCs, but are also found in LMC/SMC clusters as young as 2 Gyrs. This rules out formation mechanisms that were unique to the high redshift Universe. More generally, we find that GCs in dwarfs are, on average, more metal-poor than those in larger galaxies, although no clusters more metal-poor than [Fe/H]\approx-2.5 have been found so far. Overall, a large fraction of the most metal-poor stars in dwarf galaxies tend to belong to GCs. This has important implications for scenarios that invoke heavy cluster mass loss to account for the large fractions of enriched "second-generation" stars in GCs, as well as for globular cluster disruption and its contribution to the field star populations in halos in general.

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Galactic winds are commonly observed in starburst galaxies and play an important role in modelling realistic galaxies. However, the mechanism that drives these winds remains poorly understood. Cosmic rays provide one possibility to launch the outflows. To improve our understanding of cosmic ray driven winds, I use a set of hydrodynamic simulations of isolated galaxies to study how the wind properties change with halo mass.

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Circumstellar disks are likely gravitationally unstable early in their lifetimes when massive and still accreting from their surrounding envelopes. These gravitational instabilities will form spirals and possibly fragment into dense clumps of gas which may result in gas giant planets and low-mass stars. My research uses hydrodynamic simulations to focus on what disk conditions result in the formation of planets and do so over various resolutions to ensure convergence.

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Protoplanetary disks are the birthplaces of planets but the spatial resolution at long wavelengths has so far been insufficient to resolve the critical 5-30 AU region. The Atacama Large Millimeter Array (ALMA) now allows us to zoom in to nearby disks and determine the physical and chemical structure associated with planet formation. This talk will provide examples of recent work on observations and models of protoplanetary disks in various stages of evolution. Surveys of large numbers of disks provide insight into typical masses and sizes, revealing surprisingly weak gas emission. Special attention will be given to transitional disks, which are a subset of disks with evidence for sharp-rimmed cavities (gaps or holes). They are the best candidate sources for harboring just-formed giant planets. ALMA allows imaging of both the gas and dust in these disks, providing constraints on the properties of any young planets. Some prospects for JWST will be mentioned.

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After two decades of flying through space and more than 13 years orbiting Saturn, the journey of the Cassini spacecraft ended in mid-September with an intended crash into the atmosphere of the ring planet. The months before brought an absolute highlight: In the "Grand Finale" of the mission, Cassini crossed the narrow gap between the rings and the atmosphere of the planet 22 times. What has the spacecraft experienced in these 20 years and especially in the Grand Finale, what have we learned about Saturn, its rings and the numerous moons? After having worked with Cassini throughout its entire journey, I will give you a small insight into the most interesting and exciting events and results, garnished with images of huge storms, complex ring structures, intricate tectonics, Germany-sized seas of methane, water eruption "plumes" hundreds of kilometers high, gigantic mountains, objects that look like flying saucers, or in short: pictures of Saturn, its rings and its moons.

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The satellites of the Milky Way and Andromeda represent the smallest galaxies we can observe in our Universe. Because of the large range of mass and spatial scales it is quite challenging to resolve such objects properly in simulations. I will present a new hybrid method that we used to investigate satellites of Milky Way mass halos in hydrodynamical simulations and the effects of the satellite-host galaxy interaction that we can observe therein.

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I present observations with the MUSE instrument at the ESO-VLT that reveal the ubiquitous presence of extended Ly-alpha emitting envelopes around individual normal (non-AGN) galaxies at redshifts z > 3. These haloes are larger by factors of ~3-20 than the corresponding rest-frame UV continuum sources as seen by HST. Between ~20% and >~95% of the observed total Ly-alpha flux comes from the extended halo component. At the sensitivity level provided by MUSE, a large fraction of the field of view is actually covered with Ly alpha emission from redshifts 3 < z < 6, and I present a spectacular colour image visualising the "Sky in Ly-alpha". Our observations provide direct insights into the spatial distribution of at least partly neutral gas in the circumgalactic medium of low to intermediate mass galaxies at z > 3. I also discuss some implications for the demographics of high-redshift galaxies.

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