Vorträge, Seminare, Ereignisse

Abstract
Because this weeks speaker got ill we have to cancel the colloquium.

Abstract
A good number of massive intermediate age (around a few Gyr) metal-poor star clusters in the Magellanic Clouds (MCs) show extended main-sequence turn-off, that can not be explained by photometric errors or stellar binarity. Kron 3 is one such cluster, located to the west of the main body of the Small Magellanic Cloud which has a range of estimated radius as well as age. In this study, we have demonstrated the power of UVIT- HST- Gaia-VISTA combination to study star clusters in the MCs. We take advantage of the resolution of the HST in the central region of the cluster and the coverage of Gaia and VISTA for outer region, to combine with the UVIT data, where we have used photometry in the NUV band with a large spatial coverage. We have estimated the radius of the cluster Kron 3 as 2.0 arcmin from the UVIT and Gaia data. For the first time, we report the identification of NUV bright red clump (RC) stars and the extension of RC stars over two magnitudes both in colour and magnitude axis in NUV vs (NUV - optical) CMD. We find that the extension of RC is an intrinsic property of the cluster and not due to field star contamination. With the help of theoretical isochrones, we suggest that Kron 3 exhibits multiple stellar population with a possible age range of 6-8 Gyr and a metallicity range of [Fe/H] = -2.0 to -1.0. Theoretical spectral fits to spectral energy distribution of RC stars confirm the metallicity spread among the RC stars. The temperature of RC stars falls in a very small range of 5000-5500 K, indicating that these are indeed RC stars. We suggest that a spectroscopic study of RC stars can throw more light on the metallicity spread present in the cluster.

Abstract
It is now well established that Globular Clusters (GCs) in the Milky Way exhibit complex, multiple stellar populations that go alongside with variations in the abundances of certain chemical elements. Here I will present our results from a Stromgren-photometry study in old GCs in the LMC that indicate that all but one object exhibit no evidence for any significant metallicity spread.

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

Abstract
TBD

Abstract
Observations show that large (~100~pc) and massive (~10^5 Msun) filaments, known as giant molecular filaments (GMFs), may be linked to galactic dynamics and trace the gravitational mid-plane in the Milky Way (MW). They are the largest coherent gas structures and a central building block of the Milky Way. Yet our understanding of GMFs is still poor, limited to estimates of their occurrence, gas masses and lengths. In this talk, I will present our studies of two GMFs (GMF38a and GMF54), which are observed with different gas tracers (atomic and molecular). We studied the cold neutral media of atomic hydrogen via HI self-absorption towards the ~200pc long filament GMF38a, and compared the kinematics and column density distribution of the atomic hydrogen to the molecular gas (CO). The column density probability density functions (N-PDFs) show that the atomic gas is dominated by turbulent motions, whereas the N-PDF of the molecular gas shows a power-law tail that indicates gravitational collapse. We further mapped the dense gas of GMF54 (~45pc) with the IRAM 30m telescope, covering the common dense gas tracers HCN, HNC, HCO+ and their 13C isotopologues, as well as the cold dense gas tracer N2H+. Combining the complementary 13CO data, we studied and compared the kinematics and the density distribution of GMFs over an order of magnitude in density.

Abstract
The High Altitude Water Cherenkov (HAWC) observatory in Mexico (4100 m .a.s.l.) has just finished its first year of data taking and produced some interesting results. Because of its wide field of view it is currently a unique detector to study extended sources (>2 degree) at energies above a few TeV. One of the largest structures in the gamma-ray sky are the so-called Fermi bubbles, extending to the North and South from the milky way centre. At energies around 1TeV, and above, the HAWC observatory is in a unique position to make observations (or constrain the flux) of the bubbles. From the results of HAWC, modelling of the bubbles with a focus on the energy range above 500 GeV and improving the HAWC event reconstruction, both energy accuracy and flux sensitivity are expected to improve.

Abstract
I will use realistic N-body chemo-dynamic simulations to provide understanding about the formation and evolution of galactic discs and of their structures. After a general introduction, I will focus on a few specific points, as e.g. the influence of bars, a comparison of the evolution and properties of the thin and the thick discs, etc. My approach will include not only morphology, kinematics and dynamics, but also stellar populations, defined by their ages and chemical abundances. I will, whenever possible, make comparisons with the Milky Way.

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

Abstract
The space age of asteroseismology brought tremendous progress in the
observational probing of stellar interiors. In this seminar, we first explain
for the non expert how asteroseismology allows to deduce the interior physics of
stars at a level that is impossible to reach in any other way. We summarise the
great asset of asteroseismic sizing, weighing, and ageing of low-mass stars,
with applications to exoplanetary science and galactic archeology. Further, we
focus on the capability to derive the interior rotation properties of various
types of stars and discuss the implications for the theory of angular momentum
transport. We also touch upon recent findings on chemical mixing in the deep
interior of single and binary stars with a convective core. We end the talk by
highlighting the major opportunities from combining space asteroseismology and
astrometry with ground-based spectroscopy of large ensembles of stars in the
Milky Way and in the Magellanic Clouds.

Abstract
Recent molecular line observations with ALMA in several nearby Seyferts have revealed the existence of molecular tori, and the nature of gas flows at 10-20pc scale. At 100pc scale, or kpc-scale, previous work on gravitational torques had shown that only about one third of Seyfert galaxies experienced molecular inflow and central fueling, while in most cases the gas was stalled in rings. At higher resolution, i.e. 10-20pc scale, it is possible now to see in some cases AGN fueling due to nuclear trailing spirals, influenced by the black hole potential. This brings smoking gun evidence for nuclear fueling. In our sample galaxies, the angular resolution of up to 80mas allows us to reach the BH-zone of influence and the BH mass can be derived more directly than with the M-sigma relation.

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