Vorträge, Seminare, Ereignisse

Abstract
Bayesian inference (within or outside cosmology) bears many conceptual analogies to statistical mechanics. Quantities like thermal energy, entropy, partition functions and thermodynamic potentials integrate naturally into concepts of Bayesian inference. I hope to illustrate what one can learn from statistical mechanics for the purpose of Bayesian inference, and illustrate these with examples from cosmology.

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I present the progress so far of the Sloan Digital Sky
Survey V (SDSS-V) and the new view it yields of the Milky Way, of its
environment, and of active galactic nuclei. I will review the new
hardware and capabilities built for SDSS-V, which obtain lower
overheads allowing more rapid coverage of sky, yield new time-domain
capabilities, and enable ultra-wide integral field spectroscopy. I
will discuss some of SDSS-V's recent results on Galactic structure,
the interstellar medium, and the accretion processes of supermassive
black holes.

Abstract
In its three years of science operations, JWST has revolutionized our understanding of the early Universe. Arguably the most impressive leap forward has come from the NIRSpec instrument, providing a detailed view of the physical processes – star formation, feedback, and the growth of massive black holes – that shaped the faintest, reddest, and most distant galaxies. Among a wealth of discoveries, one overarching surprise has emerged: galaxies in the early Universe formed and matured extremely fast. Massive galaxies with old stellar populations exist already in the first billion years of the Universe, with some showing morphologies reminiscent of our own Milky Way, while others host unexpectedly massive black holes. The potential implications on galaxy formation models are profound, as current models struggle to reproduce such rapid galaxy assembly. I will present an overview of key extragalactic surveys from JWST/NIRSpec, focusing on the major discoveries that they have enabled and the challenges that remain.

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I will present our recent work using quasars and radio galaxies to challenge the cosmological principle — the foundation of modern cosmology. This principle asserts that the universe is isotropic and homogeneous, yet the Cosmic Microwave Background (CMB) reveals a strong dipole, attributed to our motion relative to the local Hubble flow. This motion should be imprinted on other observables, and I identify a dipole in large-scale surveys of cosmological sources. Whilst there is general agreement in the dipole direction, the amplitude remains at odds with expectations from the CMB. I will explore possible explanations for these tensions and consider whether a fundamental shift in our cosmological understanding is on the horizon.

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Magnetic fields permeate nearly every astrophysical environment, from planets and stars to galaxies and galaxy clusters. In these cosmologically overdense regions, magnetic fields are thought to arise primarily from magnetohydrodynamic (MHD) dynamos. These mechanisms convert turbulent kinetic energy into magnetic energy through the stretching and twisting of field lines. In the first part of this talk, I will present recent advances in our understanding of MHD dynamos. In the second part, I will focus on the vast underdense regions of space, cosmic voids, where blazar observations have revealed the existence of magnetic fields. As voids lack turbulence and therefore the energy source of classical dynamos, these large-scale magnetic fields likely originate in the very early Universe shortly after the Big Bang and therefore offer a unique window into fundamental physics. I will outline key theoretical models of magnetogenesis and present new insights in the pre-recombination evolution of these primordial magnetic fields from state-of-the-art numerical simulations. To arrange a visit with the speaker during the visit, please contact their host: Philipp Girichidis

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Episodes of supermassive black hole growth are known as “active galactic nuclei” (AGN). These are crucial periods in the life cycle of all massive galaxies. One way in which AGN can influence galaxies, is by driving multi-phase outflows of gas. However, the orders-of-magnitude ranges in spatial, temporal, and temperature scales makes this a very challenging process to constrain observationally. Questions remain on the properties, potential impact, and physical drivers of these outflows. I will present our work exploring these questions by combining cosmological simulations, high-resolution simulations of individual galaxies, and multi-wavelength observations. The power of this combined approach is understanding how specific observational experiments can (or cannot) test specific aspects of different theoretical models. Indeed, I will show how some published observational evidence that is in apparent contradiction with models, can be explained away once accounting for missing/unknown information in the data, and with a clearer understanding of which measurements can be robustly compared to simulations. I will finish by presenting new observational results highlighting a connection between dust, radio emission, and AGN outflows that now requires a physical explanation. To arrange a visit with the speaker during the visit, please contact their host: Marco Alban (ARI)

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I will carefully explain what the published astrometric Gaia data for any given star mean in detail, and how they can be used in practice. In particular, I will elaborate on the precision estimates and on the quality/reliability flags given for each star in the Gaia data releases. In addition, I will give a preview of the forthcoming epoch astrometry data, i.e. of the individual astrometric measurements for all Gaia sources, to be published for the first time in Gaia DR4 in 2026. Since Gaia mainly does one-dimensional measurements, the structure and usage of these epoch data are not intuitive for an astronomer.

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Massive stars drive galactic chemical evolution and are precursors to compact objects and gravitational wave sources. Our understanding of massive stars relies on matching observations to detailed synthetic observables computed using methods that model the star's atmosphere and winds. Traditionally, most modeling efforts of hot massive stars presume 1D spherically symmetric, steady-state atmospheres. These simplifications allow for more computational resources to be spent on accurate NLTE radiative transfer calculations. However, newer models suggest that these simplifications are not sufficiently accurate for stars at the higher end of the mass scale. In reality, multi-dimensional effects create turbulent regions with complex density and velocity structures within both the atmosphere and wind of the star. Our new method simplifies the complex NLTE radiative transfer but incorporates these multi-dimensional, time-dependent dynamics. This approach allows us to capture complex behaviors that have implications for the general atmospheric and wind structure. Applying our method has yielded several successful results. After explaining our methodology, in this talk I will highlight how multi-D models explain the winds of WR stars and predict macro-turbulent broadening of O-stars.

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To arrange a visit with the speaker during the visit, please contact their host: Fabian Schneider

Abstract
This talk has two topics. The first part is about planetary systems in star clusters. Several tens of planetary systems, including our Solar System, contain both planets and debris structures. Most stars are believed to be born in clustered environments, such as in star clusters. In such environments, debris discs evolve through interactions with stellar neighbours and planets. I use gravitational N-body simulations to investigate how the joint effect of star cluster environments and planets affects the dynamical evolution and stability of debris discs. I focus on how (i) the presence of a planet, (ii) the density of the star cluster, and (iii) the orbit of host stars within the cluster affect the stability and evolution of debris discs, as well as the characteristics of escaping particles and remaining discs. The second part of my talk is about globular clusters. They are abundant in galactic disks and spheroids, serve as ideal laboratories for studying stellar evolution alongside Newtonian and relativistic dynamics. The previous study of Dragon-II (Arca Sedda et al. 2023) successfully revealed astrophysical details of these dynamical systems, including gravitational wave signals from compact object mergers that would be measured by LIGO/Virgo/KAGRA. As a continuation of DRAGON-II, I present the DRAGON-III project and report on its preliminary results, which focuses on the simulations of million-body globular clusters and million-body nuclear clusters over 10 Gyr.

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KoCo Signature Speaker

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Binary interactions shape the evolution of the most massive stars, leading to significant deviations from the evolutionary pathways possible in single star evolution. These processes impact the universe at large scales and result in high energy events such as peculiar supernovae and gravitational wave sources. To understand these outcomes, it is important to assess binary evolution in early stages ranging from pre-interaction, roche-lobe overflow and post-interaction phases. I will discuss the current progress in our understanding of mass-transferring binaries, covering the impact of this process on the donor star (with the possible production of a stripped star), as well as the response of its companion. Of particular importance in recent years is the identification of bloated stripped stars caught immediately after interaction which provides a snapshot of the end-states of mass transfer, and I will discuss how their properties constrain orbital evolution and the efficiency of mass transfer. I will also emphasize that many of the uncertain processes in massive binary star evolution can also be assessed through the study of intermediate mass systems, for which the physics in early evolutionary phases does not differ significantly. To arrange a visit with the speaker during the visit, please contact their host: Jaime Villaseñor (MPIA)

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Giant low surface brightness galaxies (gLSBGs) have the largest discs in the Universe with the radii up to 130 kpc. The formation of such enormous discs is a stress-test for the hierarchical galaxy formation paradigm and without clarifying it we cannot paint a coherent picture of galaxy evolution. In the talk I will give the answers to the following questions. How rare are gLSBGs? What are the formation scenarios of gLSBGs? And how does it all correspond to the results of modern cosmological simulations? These answers are based on both in-depth study of 8 gLSBGs, including the results of our deep spectroscopic and photometric observations, HI data collected in the framework of our observing programs and complemented by archival datasets. Finally, we used deep optical images from HSC Subaru Strategic Program and publicly available redshift catalogs, estimated the volume density of gLSBGs in the local Universe and compared it to state-of- the-art numerical simulations.

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To arrange a visit with the speaker during the visit, please contact their host: Dominika Wylezalek

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DRAGON star cluster simulations have provided the first fully realistic long-term simulations of globular star clusters, have reproduced LIGO/Virgo observed binary black hole mergers, and are now entering into the next phase to simulate more massive, young and nuclear star clusters. They are based on the direct N-body simulation code Nbody6++GPU. In the talk an introduction and overview to direct N-body simulation and DRAGON simulations is given. Two current new applications are then shown, first initially very dense star clusters which form quickly an intermediate mass black hole of order 50.000 solar masses, which could be a seed for massive black holes in the early universe. Second, a still ongoing project is discussed, in which an already pre-existing supermassive black hole in a nuclear star cluster is followed, how it tidally disrupts stars, and captures low-mass stars, white dwarfs, neutron stars and stellar mass black holes.

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