Events, Seminars, Talks

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James Webb has revealed a surprisingly high number of bright galaxies at high redshift. This poses severe challenges to our current understanding of galaxy formation. I will report on recent advances in modeling the high redshift universe using cosmological simulations featuring radiation and magneto-hydrodynamics. A key feature of these models is the important role played by subgrid models for star formation and feedback. I will show how such models can explain the extreme properties of the interstellar medium in these early galaxies and how they impact their global properties. To arrange a visit with the speaker during the visit, please contact their host: Fabian Schneider (HITS)

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Simulating galaxies from dwarfs to clusters — the diverse and far-reaching effects of super massive black holes over the past two decades, it has become clear that energetic feedback from supermassive black holes (SMBHs) is a fundamental ingredient in our models of galaxy formation and evolution. From quenching star formation in massive galaxies to regulating the properties of the circumgalactic and intracluster media, SMBH-driven processes manifest across a wide range of scales and environments. In this talk, I will present recent advances in our understanding of the diverse and far-reaching effects of SMBH feedback, as revealed by state-of-the-art cosmological simulations. I will begin by introducing the simulations themselves — in particular, the IllustrisTNG project, which I have co-led and developed, and which has become a benchmark in extragalactic modeling for both observers and theorists alike. Using results from the IllustrisTNG simulation suite and its successors, I will demonstrate how a unified set of physical models can simultaneously reproduce realistic galaxy populations while predicting the thermodynamical, ionization, and chemical properties of cosmic gas on halo and intergalactic scales. I will highlight how feedback leaves its most direct and distinctive imprints not on the stars, but on the surrounding gas — influencing its temperature, kinematics, metal enrichment, and X-ray emission. In particular, I will contrast the effects of SMBH feedback in star-forming versus quiescent galaxies, highlighting how their gaseous atmospheres bear the signatures of these energetic processes. I will conclude by providing a diversity of insights and predictions for the gaseous haloes of galaxies spanning five orders of magnitude in mass, illustrating how SMBH feedback shapes the circumgalactic, intragroup, and intracluster media.

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Over the past decade, numerous population studies have been carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) to investigate the fundamental properties of protoplanetary disks around young stars. These studies have primarily focused on low-mass stars, with relatively few disks observed around the more massive Herbig stars, even though the latter are important formation sites of giant exoplanets. In this presentation, I will take you through the journey that began at the start of my PhD: a quest to construct the first volume-complete millimeter survey of Herbig disks. Along the way, I will present the dust and gas masses obtained from ALMA archival data, discuss how these compare to the population of disks around lower mass stars, and show the first complete millimeter study of Herbig disks within a single star-forming region. Luckily, this story seems to have a happy ending, as I'll offer the first glimpse into a complete millimeter survey of all Herbig disks within 1kpc, and the new questions it starts to raise.

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To arrange a visit with the speaker during the visit, please contact their host: Laura Kreidberg

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To arrange a visit with the speaker during the visit, please contact their host: Kees Dullemond (ITA)

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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 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

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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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