Events, Seminars, Talks

Abstract
The origin of supermassive black holes in galaxy centers remains one of the key unanswered questions in modern astrophysics. If SMBH seeding occurs at high redshift, the outcomes for the black hole (BH) population in the local Universe will differ significantly depending on whether the seeds are light (stellar-mass) or heavy (hundreds of thousands solar masses). A recently discovered population of active nuclear intermediate-mass black holes (IMBHs, 1e2<MBH<2e5) in the local Universe seems to favor the light-seed scenario. However, a disproportionately large number of IMBHs accreting close to or above the Eddington limit pose a new question: Are we just lucky to see short bursts of activity or do they actually grow fast after many billions of years of quiescence? An unexpected solution comes from in-depth studies of nearby (z<0.3) analogs to Little Red Dots, an abundant population of point-like sources discovered in JWST surveys at high redshifts. These sources resemble AGN but lack some important features (X-ray, high-ionization emission lines, mid-IR excess). Our analysis of high-quality spectra of 10 nearby Little Red Dots suggests that they are powered by supermassive (2e4-3e5 MSun) stars with dust envelopes living in massive young star clusters hosted by metal-poor Blue Compact Dwarf Galaxies. Dedicated models of metal-enriched (i.e. Z=1e-4...1e-2 ZSun; non-PopIII) SMSs reproduce the observed spectral features of local LRDs, such as photospheric absorption lines, stellar winds with high mass loss rates estimated from P-Cygni profiles, and broad exponential wings of hydrogen and helium lines that mimic AGN, which originate from Thomson scattering. SMSs are expected to live for 0.5-2 Myr after which they should collapse into IMBHs (i.e., massive seeds) that will accrete the surrounding (fallback) material at the Eddington rate for several million years. This exotic scenario provides a self-consistent explanation for the observed properties of low-to-high redshift LRDs and nearby active IMBHs. It also resolves the high-redshift quasar puzzle: z=7 MBH=1e9 MSun SMBHs become feasible because the BH seeds can be heavy, however the seeding can happen as late in the Universe's history as z=0 even though the chances of achieving the conditions necessary for SMSs to form decrease over time because they require a low-metallicity environment.

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Stellar populations preserve a record of galaxies' integrated baryon and metal cycles, governed by star formation, metal production and interaction with the surrounding medium. By decoding stellar population properties from galaxy spectra—the "archaeological approach"— fundamental scaling relations have been uncovered linking stellar ages and chemical abundances to galaxy mass, structure, and dynamics. These scaling relations are benchmarks for galaxy evolution models and provide statistical “demographic” distributions at a given epoch. Extending the archaeological approach to earlier cosmic epochs is essential to disentangle individual evolutionary paths from population evolution and eventually allow a statistical connection between progenitors and descendants. The intermediate-redshift regime (the last 8 Gyr) is particularly critical: massive early-formed systems coexist with ongoing quenching in lower-mass galaxies, while all galaxies continue evolving dynamically, morphologically, and chemically. Recent deep spectroscopic surveys now enable archaeological studies at these earlier epochs, complementing traditional number density evolution analyses. I will give an overview of our results constraining stellar population properties from low-redshift spectroscopic surveys and their dependence on mass, star formation activity, and environment. I will then discuss how these scaling relations evolve over the last 6 billion years, as revealed by the LEGA-C survey. This type of analysis provides a foundation for interpreting archaeological investigations of galaxies over a continuous span of cosmic time, as will be obtained from WEAVE, 4MOST, MOONS deep spectroscopic surveys, bridging local and higher-redshift studies. To arrange a visit with the speaker during the visit, please contact their host: Anna Pasquali

Abstract
The origin of intermediate-mass black holes remains an open question. Massive and dense stellar environments are proposed as their potential birthplaces. In this work, we investigate the role of stellar collisions in forming very massive stars (VMSs) that subsequently collapse into black hole (BH) seeds. Using both direct N-body and Monte Carlo simulations, our models reveal a critical mass threshold above which collisions become highly efficient, allowing a significant fraction of the cluster mass to be funneled into a central massive object. We find that VMSs can form and collapse into BH seeds within a few Myr in extremely dense clusters. These results support stellar collisions as a robust pathway for early BH growth in dense environments and provide a theoretical framework for interpreting JWST observations of compact high-redshift clusters.

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Well-characterised multiple-exoplanet systems are particularly valuable because their physical and dynamical architectures preserve a fossil record of their formation and subsequent dynamical evolution. In strongly interacting systems composed of massive Jovian planets detected through radial velocities (RVs) and transit timing variations (TTVs), the mutual gravitational interactions are strong enough to constrain the system architecture with high precision through N-body dynamical modelling. Such models recover the time-dependent osculating orbital elements that more accurately describe the true configuration and dynamical state of the system. Reproducing the observed dynamical architecture of these systems enables reverse engineering of the initial conditions under which they formed. In particular, planetary systems trapped in mean-motion resonances (MMRs) retain a “memory” of the migration process, including the possible migration rate during the proto-planetary disk phase, the excitation of eccentricity, and disk-planet interactions at the moment of capture. In this talk, I will present the cascade from observations to detailed dynamical modelling of the data, and the following migration simulations. I will highlight several exceptionally well-characterised giant planet systems that provide important insights into the mechanisms shaping planetary systems during their formation and early evolution.

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Barred galaxies tend to exhibit a dense and highly star forming region at their center, called Central Molecular Zone (CMZ). Gas dynamics and star formation in CMZs are fundamentally shaped by the complex gas motions and extreme conditions of the region. Leveraging dedicated radiation-MHD simulations we compare ISM properties and star formation in such a region to a Solar-neighborhood type of environment, highlighting and understanding differences. Importantly, strong shear, short orbital timescales and frequent cloud collisions, tend to decouple stars from their surrounding gas early in their evolution, when feedback is still active. This has important consequences on how stellar feedback couples back to the ISM and changes the classical picture of how HII regions and superbubbles evolve.

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Solar evolutionary models are key calibrators of stellar evolution theory. The high quality of observational constraints and the fact that the solar mass, radius and age are known with high accuracy and precision allows to use the Sun as a true laboratory of fundamental physics. Over the last decades, most solar modellers relied on a simplified framework to model the evolution of the Sun, called the Standard Solar Model framework, neglecting dynamical processes and designed for large-scale computations. These models encountered widespread success (see e.g. Model S from Christensen-Dalsgaard et al. 1996), but both helioseismic and asteroseismic data have shown the limitations of standard models. In the last months of the preparation of the PLATO mission (to be launched in early 2027), the accuracy of stellar models comes as a crucial question that actually links all our recent progress in asteroseismology back to one burning question: how well do we know our Sun? To arrange a visit with the speaker during the visit, please contact their host: Maria Bergemann

Abstract
Galactic outflows are inherently multiphase, spanning an extraordinary range of physical conditions—from cool, ionized gas traced by Halpha emission to hot, volume-filling plasma observable in X-rays. While recent observations have significantly improved constraints on how gas is transported in these winds, their overall energy budget—and, crucially, the coupling between different gas phases—remains poorly understood. Addressing these open questions requires a combination of multi-wavelength observations and numerical simulations capable of resolving the full complexity of the interstellar medium (ISM). In this talk, I will present a suite of state-of-the-art magnetohydrodynamic (MHD) simulations that capture the structure and origin of multiphase galactic winds across a diverse range of environments. I will begin with low-redshift simulations achieving a gas mass resolution of 1 Msun and a spatial resolution of 0.1 pc, and demonstrate how these results connect to observations of nearby dwarf galaxies such as Leo P, WLM, the SMC, and the LMC. In the second part, I will highlight recent advances in modeling galactic winds at ultra-high redshifts (z ∼ 10), performed at comparable resolution. I will conclude by discussing the implications for the baryon cycle and the phase structure of early galaxies, including observational predictions that can be tested with JWST using ionized gas tracers (e.g., Halpha and [O III]) and with ALMA via cold and molecular gas tracers such as [C II] and CO. Together, these simulations provide new insight into the multiphase structure of galactic winds across cosmic time, and establish a framework for directly connecting high-redshift galaxies to their local, low-redshift analogues.

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Understanding the origin of the elements is one of the major challenges of modern astrophysics. Ultraviolet (UV) spectroscopy of metal-poor stars provides access to many absorption lines of elements and species that are otherwise undetectable in optical or infrared spectra. I will show how UV spectra collected with the Hubble Space Telescope have expanded stellar chemical inventories to more than 65 elements per star and identified signatures associated with r-process transuranic fission fragments. I will also show how UV spectroscopy with the ANDES instrument on the Extremely Large Telescope and the proposed Habitable Worlds Observatory mission could revolutionize our understanding of the first stars in the decades ahead. To arrange a visit with the speaker during the visit, please contact their host: Norbert Christlieb

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Recent JWST and ALMA observations have revealed that a substantial fraction of galaxies at very high redshift are rotationally supported disks. Many of these systems host kinematically cold, or “settled,” gaseous disks, a finding that stands in tension with the expectations of many galaxy-formation models. In this seminar, I will present an overview of cosmological simulations of disk-galaxy formation. I will discuss the properties of gas-rich, high-redshift disks, their turbulent nature, and how these early phases of disk assembly connect to spectroscopic and astrometric observations of the Milky Way. Finally, I will show new results indicating that turbulent, early-Universe galaxies are more susceptible to global gravitational instabilities, such as bar formation, than previously thought. These results carry important implications for the rapid buildup of galactic bulges and the fueling of central black holes. To arrange a visit with the speaker during the visit, please contact their host: Eva Schinnerer

Abstract
In the LambdaCDM model, the large-scale matter distribution of the Universe regulates the hierarchical assembly of dark matter halos. At the centers of these halos, intricate baryonic processes unfold (e.g., gas accretion, star formation, chemical enrichment, stellar and black hole feedback), giving rise to observed galaxies. Yet, how the effect of halo and large-scale environment propagates into the baryonic physics of galaxies remains far from being fully understood. In this work, we assess the effect of different local and large-scale environmental metrics on stellar population properties and star formation timescales derived from rest-frame optical absorption spectra and photometry of massive LEGA-C galaxies at 0.6 < z < 1. We report that overdensity derived from reconstructed density field is the main environmental metric driving the stellar population properties of massive galaxies at intermediate redshifts. We observe that galaxies of a given stellar mass in overdense regions are older and form earlier on and over shorter time-scales than the ones in underdense regions. What is more interesting, these trends remain after accounting for the galaxies’ hierarchy within groups and clusters and their position in the large-scale cosmic web. Our results suggest that the initial conditions of the matter density field have a profound effect on the quenching and star formation histories of galaxies, leaving an imprint on observed galaxy stellar populations and scaling relations that are already in place at intermediate redshifts. Upcoming large spectroscopic surveys with large statistics and exquisite cosmic web characterizations at intermediate redshifts are key to further bring light into this matter. In this context, future dedicated wide-field spectroscopic facilities, such as the WST concept, could revolutionize our view of how the cosmological environment of galaxies drives their formation.

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Active galactic nuclei are mostly powered by inflow through accretion disks onto central supermassive black holes. Beyond a few times their Schwarzschild radius, gravitational instability in these disks leads to self-regulated formation and evolution of massive stars which chemically enrich their neighborhood along with stellar-mass black holes. These compact remnants are captured by coexisting massive main sequence stars, form close binaries, readily merge, and excite intense gravitational waves with potentially observable electromagnetic signatures. The massive stars' migration, with or without black hole cores, efficiently transporting mass regardless of the Eddington limit and promoting the rapid growth of supermassive black holes in the early Universe. Analogous physical processes are also relevant in the context of planet formation in protostellar disks. They account for the persistent super-solar metallicity, especially in Nitrogen and iron, inferred from broad emission lines of high and low redshift AGNs. To arrange a visit with the speaker during the visit, please contact their host: Haochang Jiang

Abstract
Microlensing has long been a powerful tool for studying dark objects, but its full potential has yet to be unlocked. Typically, information about the lens comes solely from the light curve, which is subject to strong degeneracies. In addition to increasing the observed brightness of the background star, microlensing also produces tiny shifts in its apparent position — a phenomenon known as astrometric microlensing. This effect can break degeneracies and, crucially, allows for direct measurement of the lens mass. Thanks to recent advances in astrometry, detecting these subtle shifts is now becoming possible. Until now, astrometric microlensing has yielded only a handful of detections, which required expensive, targeted follow-up. This year, the field will reach two major milestones. The Roman Space Telescope, the first space mission designed for microlensing observations, is set to launch this autumn. Later in the year, Gaia Data Release 4 will, for the first time, provide astrometric time-series data for nearly two billion sources. These new datasets will finally make it possible to build a systematic census of isolated dark stellar remnants in the Milky Way. Using detailed simulations, we have explored how to identify dark remnants, optimize the use of observation and analysis resources, and extract masses from astrometric signals. I will discuss these efforts and showcase the exciting developments that lie ahead in 2026.

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Modern astronomy has entered a regime in which the central challenge is no longer data acquisition, but data integration. Billions of stars are now observed across (phase-)space, frequency, and time, yet each modality captures only a partial view of the processes that shape our Galaxy. The next frontier in Galactic archaeology lies in unifying these dimensions into a coherent framework. In this talk, I will present new data-science methods that integrate spectroscopy, asteroseismology, and astrometry to infer precise stellar ages, chemical abundances, and dynamical histories across the Milky Way. I will particularly highlight scalable time-domain techniques for extracting stellar oscillations from large survey data, transforming time-series observations into powerful constraints on Galactic evolution. These approaches quantify the efficiency with which disk stars lose dynamical memory, and show that the disk’s chemical bimodality likely reflects a merger-driven origin linked to the formation of the Galactic bar. I will conclude by outlining a path toward constructing a unified, multi-dimensional map of the Milky Way, spanning spatial, chemical, dynamical, and temporal dimensions, using datasets such as Euclid and Rubin LSST, positioning time-domain and multi-modal data science at the centre of Galactic evolution studies.

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Ultra-hot Jupiters (UHJs) have become prime targets for atmospheric characterisation. Owing to their close-in orbits and the early spectral types of their host stars, these planets are unique laboratories for studying atmospheric physics and chemistry under extreme conditions. One of the most compelling processes to investigate in these environments is atmospheric escape, which is thought to be a major driver of the long-term evolution of planetary atmospheres across a wide range of planets. Atmospheric mass loss is powered by heating processes that are strongly influenced by non-local thermodynamic equilibrium (NLTE) effects. I will show how NLTE processes shape the physical and chemical structure of UHJ atmospheres and discuss the observational evidence supporting these results on the basis of multiple datasets. To arrange a visit with the speaker during the visit, please contact their host: Maria Bergemann

Abstract
Interacting and merging galaxies are crucial for the understanding of how galaxies assemble and evolve, moreover they also provide a unique probe of cosmological models of the universe. However, classifying galaxies as interacting remains a major challenge. With the Q1 release of Euclid a new dataset of high-quality photometric and spectroscopic data is available, enabling the assembly of a large statistical sample of galaxies undergoing mergers. To create such a sample, we make use of a novel semi-supervised machine learning tool dubbed "AnomalyMatch", designed to identify interesting objects. AnomalyMatch requires only a small number of initial labelled training samples (< 100) and allows one to find objects of interest quickly via user-in-the-loop active learning. The output of the machine learning model is used to model the distribution of mergers, relying on Bayesian analysis and Markov Chain Monte Carlo on the basis of expert labelled images. To determine its performance, the model and Bayesian Inference is evaluated on synthetic imagery stemming from the IllustrisTNG simulation where it manages to reproduce the underlying fraction of mergers. The application to a mass-complete sample of about 913k Euclid galaxies with 0.2 < z < 1.0, indicates a decreasing merger fraction evolution ~0.2 over the entire mass range, which is in contrast to some of the literature. Adopting recent estimates for observational timescales, we compare the fractional galaxy merger rate to expectations from the Illustris simulations and find that our results indicate increasing, but higher absolute galaxy merger rates than previous work. Assuming a certain distribution of pre and post-mergers in our sample recover absolute values, but flatter evolution than the Illustris simulation predicts.

Abstract
A third of all massive stars are predicted to lose their hydrogen-rich envelope through mass transfer or common envelope ejection initiated by a binary companion star. As a result, the hot and compact helium core is exposed. These "stripped stars" are the direct progenitors of hydrogen-poor supernovae and merging binary neutron stars, but they are also so hot that they should boost the ionizing output from bursty star-forming galaxies. Despite their importance, stripped stars remained, until recently, observationally unconfirmed since their predicted existence over half a century ago. We found the first set of stripped stars by combining ultraviolet and optical photometry with follow-up spectroscopy in the Magellanic Clouds. By fitting their spectra with a new grid of models, we could measure stellar properties and thus confirm that the predictions from binary evolution models are broadly consistent with observed stripped stars. To arrange a visit with the speaker during the visit, please contact their host: Jaime Villasenor

Abstract
The calculation of calendrical data has a long tradition at ARI. The annual volume "Astronomische Grundlagen für den Kalender" is edited by ARI and provides calendar publishers in Germany with fundamentals for the production of calendars. It is available open access at: https://journals.ub.uni-heidelberg.de/index.php/agfdk. In this talk I describe how it happened that eminent scientists like Leibniz and Roemer around the year 1700 worked on the task of getting the date of Easter "right" in an astronomical sense, and how this effort is connected to the history of ARI.

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

Abstract
Stellar feedback is a key driver of galaxy evolution, yet its impact on the interstellar medium remains difficult to quantify across entire galactic environments. Massive stars inject energy and momentum through radiation, stellar winds, and supernova explosions, clearing out their immediate surroundings. In the process, they sweep up and compress the ambient gas into dense, ring-like structures often associated with enhanced star formation. At the same time, competing processes such as turbulent mixing, radiation, and winds disperse this dense material, erase the surrounding enhancement, and ultimately suppress further star formation. Stellar-feedback-driven bubbles therefore both trigger and quench star formation, making them key regulators of galactic star formation histories. Using James Webb Space Telescope observations of nearby galaxies from the PHANGS collaboration, we probe this process across entire galactic disks. We identify and extract voids using a novel algorithm based on persistent homology, capable of detecting spatial structures on all scales accessible in the analyzed data. By tracking the topology of the emission, this method robustly identifies voids without imposing arbitrary intensity thresholds or geometric assumptions. Although the observations provide only a snapshot in time, the large population of detected bubbles enables a statistical reconstruction of their evolutionary sequence. We then classify the detected bubbles based on the physical properties of the voids and their surrounding medium, separating systems with enhanced shells around evacuated interiors from more evolved structures in which the central void begins to refill and the surrounding enhancement has faded. This framework traces the lifecycle of stellar-feedback-driven bubbles and their role in regulating star formation in nearby galaxies.

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To arrange a visit with the speaker during the visit, please contact their host: Hans-Walter Rix

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Stellar counter-rotation (CR), where a large fraction of stars rotate opposite to the old stellar disk, is a striking dynamical signature of past galaxy interactions. Such kinematically decoupled structures are thought to form after the accretion of gas or during fine-tuned collisions of galaxies that bring in material with opposite angular momentum. Using MaNGA survey data, we have identified over one hundred CR galaxies, but one system stands out: NGC 5717, the most massive known CR galaxy. This galaxy is a brightest cluster galaxy, morphologically resembling an elliptical with an embedded, massive disk with young stellar population and ongoing star formation. The scale and persistence of this structure in a dense environment challenge current views on how disks survive and reform after merging. In this talk, I will present the sample of such galaxies, the properties of NGC 5717, and discuss its implications for understanding disk assembly and the role of galaxy collisions in shaping CR-galaxies.

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To arrange a visit with the speaker during the visit, please contact their host: Hans-Walter Rix

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I will review the recent progress that we have made in understanding nucleosynthesis in the Universe and the accretion history of the Milky Way through chemodynamical analysis of the stellar halo. Over the last five years, we have been complementing the discoveries from the Gaia mission with high-resolution spectroscopic follow-ups of halo stars. These studies have provided detailed chemical abundances of stars in stellar streams, revealing the origins of these streams. There have also been serendipitous discoveries during these follow-ups leading to new insights into broader topics in astrophysics, such as the discovery of Li-rich stars in stellar streams and the association of a 33 Msun solar mass black hole with a stellar stream. After reviewing these discoveries, I will discuss the future prospects of this field in light of the recently started spectroscopic surveys, namely 4MOST and WEAVE, hopefully with initial results from them by the time of the colloquium. Finally, I will briefly introduce the next generation spectroscopic facilities that are currently being discussed. In addition to these scientific topics, I will also share my personal experience of being a Gliese fellow at ARI for three years and how it has impacted my career.

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To arrange a visit with the speaker during the visit, please contact their host: Markus Hundertmark

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Westerlund 1 (Wd 1) is the most massive young stellar cluster in the Galaxy, and hosts a uniquely numerous, diverse and nearby population of evolved massive stars including 24 Wolf-Rayet stars, several yellow and red supergiants, a luminous blue variable and over 100 OB supergiants. In this talk I will report on the supergiant B[e] star Wd1-9, which we now know to be a recently stripped massive binary system deeply embedded in a dense rotating circumstellar medium and bipolar photoevaporating outflow reminiscent of a protoplanetary disk wind. I will also detail how hydrodynamic simulations of outflows from massive stars coupled with EWOCS data can constrain supergiant evolutionary pathways in clusters. I will show how a recent (~10 kyr ago) non-conservative mass transfer event can explain the unusual nebulosity around the WR+O binary system Wd1-72. I will also demonstrate how the mysterious "pillar/rat" nebula in Wd 1 could have been produced by the yellow supergiant Wd1-4 transitioning from a red supergiant in the past ~15 kyr, and how similar models of cool supergiants in clusters have the potential to robustly measure their uncertain mass-loss rates.

Abstract
Hot, massive stars are astrophysical keystones which shape their environment via mechanical and radiative feedback and chemically enrich their cosmic neighborhood since the first generation of (very) massive stars. However, turning this general textbook picture into quantitative prediction remains an ongoing challenge. Recent discoveries, such the surprisingly high metallicity and early nitrogen enrichment in high-redshift galaxies discovered by JWST, put current descriptions and modelling approaches into question, illustrating that the complex puzzle of massive stars and their interplay is everything but complete. To get a robust, quantitative understanding, decoding and predicting the light of hot massive stars marks an astrophysical key technique. Revolving around via the development and application of expanding stellar atmosphere models for hot stars, I will present a selection of recent research results from my group. A particular focus will be on star with strong stellar winds, their enigmatic cosmic role, and the challenge to get a coherent structural and evolutionary understanding of these objects. Moreover, I will present our recent discovery of an unexpected direct transition in the Wolf-Rayet regime from WN to WO subtype occuring at subsolar metallicity as well as ongoing and forthcoming efforts in further developing atmosphere modelling and quantitative spectroscopy.

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