Events, Seminars, Talks

Abstract
The centers of massive galaxies are special in many ways, not least because apparently all of them host supermassive black holes. Since the discovery of a number of relations linking the mass of this central black hole to the large scale properties of the surrounding galaxy bulge it has been suspected that the growth of the central black hole is intimately connected to the evolution of its host galaxy. However, at lower masses, and especially for bulgeless galaxies, the situation is much less clear. Interestingly, these galaxies often host massive star clusters at their centers, and unlike black holes, these nuclear star clusters provide a visible record of the accretion of stars and gas into the nucleus. I will present our ongoing observing programmes of the nearest nuclear star clusters, including the Milky Way Center, and the stripped galaxy nuclei Omega Centauri and M54 (still at the center of the Sagittarius dwarf galaxy). These observations provide important information on the formation mechanisms of nuclear star clusters, allow us to measure potential black hole masses and give clues on how black holes get to the centers of galaxies.

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Bright blazar jets have emerged as prominent sources of high-energy neutrinos, with multiple IceCube events linked to them over the past decade. Being the most direct tracer of ultrarelativistic protons, neutrinos are crucial to understand the matter and energy content of the jet. In this talk, I present recent observational findings on the neutrino-blazar connection, and attempts to quantify the hadronic jet component through the neutrino flux. High-resolution observations with radio interferometry (VLBI) play a key role in these results. Our findings indicate that ultra-relativistic protons constitute a significant, even if subdominant fraction of the total jet power. This property appears to be common among bright blazars, and should be incorporated into physical modeling of AGNs going further.

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Stellar spectroscopy is going through a revolution, changing manual labor by automated processing and artificial intelligence, and replacing human observers and single-target instruments by robots and highly multiplexed spectrographs. Yet, improvements at the fundamental physics level are happening far slower, and the most popular stellar atmosphere models employed to interpret stellar spectra are based on codes originally written in the 1970's. In this talk I will discuss recent advances and reflect on what's missing to realize actual progress in our understanding of the assembly and early evolution of the Milky Way from the study of the most primitive stars. To arrange a visit with the speaker during the visit, please contact their host: Hans Ludwig (LSW)

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Star clusters are the one of the most useful places in the universe for studies of stellar and galactic evolution. Formed when stars collapse from the same molecular cloud into a cluster, star clusters offer a unique way to study stars of a homogeneous age and chemical composition across a range of masses. In the age of the Gaia satellite, the census of star clusters in our galaxy has exploded in size - but not without also presenting a number of challenges that require new machine-learning based techniques to solve. In this talk, I will present my work so far aiming to improve many aspects of the census of galactic star clusters in the age of Gaia. After an introduction to the field, I will start by discussing our published catalogue of over 7000 clusters, which represents the largest homogeneous unduplicated catalogue of Milky Way star clusters to date. I will discuss our recently published update to the original catalogue that measures the largest ever sample of cluster masses to better define them observationally, including many surprising results like that most clusters in the galaxy appear to have the same initial mass function - but only after first correcting for selection effects. I will discuss our ongoing work on the sample’s completeness and variable star content. Finally, I will discuss future avenues for research, including upcoming data releases like Gaia DR4 and the LSST.

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I will present the latest insights into the formation pathways of early galaxies, placing them in the broader context of self-regulated galaxy growth observed both today and during cosmic noon. Beginning with new theoretical developments from the cosmological THESAN-zoom simulations, I will explore how early star formation is regulated, how galaxies evolve along the star-forming main sequence, and how their sizes increase over cosmic time. I will then connect these theoretical predictions to recent JWST observations from NIRCam and NIRSpec as part of the JADES survey – one of the most extensive observational campaigns conducted with JWST. I will highlight the diversity of galaxies at cosmic dawn (redshift z>10), where vigorous star formation and black hole growth are prevalent. Next, I will examine the role of mergers in shaping galaxy evolution and discuss the emergence of disk-like structures during the Epoch of Reionization (z=4-10). I conclude by placing observational constraints on the morphological evolution of galaxies within the framework of star formation variability, providing a comprehensive view of how early galaxies grew and evolved. To arrange a visit with the speaker during the visit, please contact their host: Richard Tuffs

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Galaxies are not isolated objects; they frequently interact with their surrounding environment, leading to significant transformations; both in the morphology and stellar populations. These interactions often leave behind low surface brightness tidal streams as remnants of stellar populations on the outskirts of galaxies, such as streams, shells, and tidal tails. The study of these remnants offers deeper insights into the structure and stellar populations of galaxies. For instance, major mergers offer unique insight into the formation and history of elliptical galaxies. We use the J-PAS survey early data release (EDR) to obtain Spectral Energy Distributions (SEDs) of a major merger with strong tidal features as a case-study (PGC 3087775). The unique feature of J-PAS is its 54 contiguous narrow-band filters covering the optical range from about 3800A to 10000A; over the northern non-Galactic sky (more than 8000 degrees squared at its end; EDR was 12 degrees squared). Given the diffuse nature and large size of galaxy mergers (in the nearby universe), Integral Field Unit (IFU) data is not practically possible. But by covering both the Balmer break, Dn4000 and precise estimate of the redward slope, J-PAS allows robust estimates of the stellar populations of the separate merger remnants. When the J-PAS SED of PGC 3087775 was fitted with CIGALE, the measured stellar population age, metallicity and mass showed a coherent physical mechanism of the mass assembly of this particular to-be elliptical galaxy. Through the non-detected emission-lines we also conclude that this is a dry major merger. The first public data release of J-PAS will be coming in the next year, covering almost 100 degrees squared; allowing the identification and application of a similar analysis on a much larger sample of merging galaxies. To do this, besides the exclusive features of J-PAS, we also need an automatic method to find and extract merger remnants from their host galaxies in wide area imaging surveys. We have defined a new non-parametric algorithm for this purpose; utilizing the azimuthal angle and magnitude of the gradient of each pixel's value to extract and expand the skeleton of the streams. This algorithm will be later applied to J-PAS (in combination with Euclid for a better spatial resolution, but poor spectral resolution!) in preparation for the latest European Space Agency (ESA) astronomical mission: ARRAKIHS.

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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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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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The field of black hole accretion is seeing a renaissance in the past 5–10 years, thanks to the advent of time domain surveys across the electromagnetic spectrum. These surveys monitor hundreds of thousands of galaxies at unprecedented cadence, revealing the secrets black holes were keeping while we weren’t watching. In this talk, I will present recent highlights on black hole accretion and growth in two parts: (1) in “Standard Accretion” events like Active Galactic Nuclei and accreting stellar-mass black holes, where we can use Reverberation Echoes to map the inflowing gas and measure the black hole spin, (2) through the discovery and characterization of exotic transients, like Tidal Disruption Events, and a new phenomenon called Quasi-Periodic Eruptions (QPEs), which have been posited as due to the presence of an orbiting stellar mass object, or EMRI. We will discuss the current state of the field, and implications for joint detections in the next decade with future space missions like AXIS, the X-ray Probe recently selected for a NASA Phase A study, and the recently adopted LISA Gravitational Wave Observatory. To arrange a visit with the speaker during the visit, please contact their host: Julien Wolf

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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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