Events, Seminars, Talks

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Abstract: Nearly 500 years ago, Nicolas Copernicus published his disruptive theory that Earth is not the center of the universe. This "Copernican demotion" has held fast over the centuries, as astronomers have learned that there is nothing particularly remarkable about Earth or even the Milky Way. In the last two decades, however, a new test of the Copernican Principle has emerged -- the discovery of an abundance of planets orbiting other stars. These discoveries allow us to put Earth in context and evaluate whether the formation, architecture, and present-day characteristics of our Solar System are in fact typical. Thanks to the revolutionary capabilities of the James Webb Space Telescope (JWST), we are finally able to study other Earth-size planets in detail, and in particular search for and characterize their atmospheres. In this talk, I will give a status report on JWST observations of rocky planets. I will cover the latest results for the iconic TRAPPIST-1 system, the study of the surface of the airless planet LHS 3844b, the search for atmospheres on lava worlds, and observations of planets in the radius valley at the boundary of rocky and gaseous worlds. Taken together, these results provide a first picture of the building blocks available for the origin of life on terrestrial planets beyond the Solar System, providing essential context for how special Earth really is.

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Understanding the formation of (exo)-planetary systems requires the combined effort of advanced computational models and high-resolution multi-wavelength observations. In my talk, I will review our current understanding of the dynamic evolution of protoplanetary disks. 3D multi-physics simulations and high- performance computing allow us to study the thermal and kinematical evolution of young circumstellar disks and planets’ birthplaces in detail. I will also highlight why the inner disk regions, especially those close to the silicate sublimation, are crucial for forming terrestrial planets.

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Take a molecular cloud, collapse it to form a star and the leftover material will form planets. Sounds easy, right? But even our own solar system is riddled with clues that forming stars and planets is a bit. more. complicated. It turns out that accreting gas to form any small object is hard. Accreting gas at the rate needed to form the Sun in a few hundred thousand years is even harder. None of this is new. What is new is the observational revolution of the last 10 years, showing us the insides of protoplanetary discs, bringing fresh clues as to how both stars and exoplanets form [seemingly, together]. This has dramatic implications for our understanding of how accretion works. I will argue that the typical pathway to form stars and planets is a violent mess, imprinted in subtle and not-so-subtle ways on disc observations and also in the leftovers from our solar system’s formation. The story is misaligned flow, accretion streamers, infall, warps and variability. If you don’t care about stars or planets but the story sounds familiar, it’s because it’s not so different for making black holes or galaxies either... To arrange a visit with the speaker during the visit, please contact their host: Mike Lau

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In this colloquium, I will start by briefly reviewing the importance of stars in the Universe. I will then discuss how we can re-create stars on computers using a range of simulations using a few hours on a laptop to months on supercomputers. I will explain how these simulations help us understand and predict the structure, fate and impact of massive stars. In particular, I will present recent work studying how mass and rotation affect their fate across cosmic times and compare the predicted black-hole mass distribution to the latest gravitational waves detections from the LKV collaboration. I will then introduce the 321D (3 to 1-dimension) loop, a framework to improve 1D stellar evolution models using very detailed 3D hydrodynamic simulations.

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Nearby galaxies observed at high spatial resolution with JWST, ALMA, and MUSE allow us to address fundamental questions about the influence of young stars on their surrounding interstellar medium (ISM), from giant molecular clouds (GMCs; 50-200 pc) to galactic scales: How far can ionizing photons travel, and what physical mechanisms favor their escape from HII regions? How do such processes shape the ISM and influence galactic evolution? I will first present constraints on the timescales and physical mechanisms associated with the evolutionary cycle of GMCs in massive star-forming galaxies from the PHANGS-JWST survey, including their dust-embedded star formation phase. We find that the embedded phase of star formation is short (< 4 Myr), hinting at a dominant role of pre-supernovae feedback. Strikingly, the duration of this phase is reduced in metal-poor galaxies, which may host a different population of HII regions associated with higher ionizing photon leakage. These results suggest that the intrinsic ISM properties and distribution around ionizing sources strongly influence the early feedback phase. I will then introduce a recently developed statistical framework that models the complex ISM geometry by combining multiple components, enabling constraints on key physical parameters, such as density, ionization parameter, and escape fraction. Finally, I will discuss how resolved and unresolved approaches complement each other, particularly for calibrating models essential to high-redshift studies.

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In the era of `Precision Cosmology' remarkable advances have been made in the determination of cosmological parameters from the Cosmic Microwave Background and Big Bang Nucleosynthesis, with spectacular concordance between these two pillars of the Standard Cosmological Model. While much exposure has been given to the impressive results from the WMAP and Planck missions, perhaps less attention has been paid to the equally striking advances made in the last ten years in the measurements of the abundances of the light elements forged in the first few minutes of our Universe history. In this talk I shall focus in particular on the determination of the primordial abundance of deuterium, in an overview that spans almost 80 years, from the first seeds of the idea sown in the 1940s to the most recent results and forward look to the era of Extremely Large Telescopes and next generation Wide Field Surveys of the sky. To arrange a visit with the speaker during the visit, please contact their host: Eduardo Banados

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Ten years after the first direct detection of gravitational waves from merging compact object binaries more than two hundred events have been observed. However, the astrophysical mechanism that forms them remains unclear. Isolated binary stars, higher-order multiples, dense star clusters, or active galactic nuclei could all be culprits and contribute to the observed population. In this talk, I will discuss how the detection of extreme outliers - such as binary mergers with orbital eccentricity or spin-orbit misalignment - can be used to constrain formation channels and to learn about the evolution of the massive progenitor stars.

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To arrange a visit with the speaker during the visit, please contact their host: Marcelo Alberto Cortes Vergara

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Over the past decade the space missions CoRoT, Kepler and TESS have revolutionised the field of asteroseismology - the study of the internal structures of stars through their global intrinsic oscillations. A particular steep increase in our knowledge has been possible in stars cooler than 6700 K, which exhibit convection in their outer layers. Oscillations are excited in these turbulent layers that allow us to probe large parts of the stars. In this talk I will present recent results of asteroseismic inferences of the stellar structure of these cool stars.

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Over the next few years, Rubin/LSST, Euclid, Roman, SKA, and other instruments will produce petascale, information?rich datasets that trace stars, galaxies, and large-scale structure with unprecedented fidelity. Hidden in these data may be regularities that point to new and unexpected physical relationships. Can we build modelling frameworks that can discover such relationships accurately, efficiently, and in forms we can interpret? I will present two complementary directions we are developing to address this question. The first, PhySO, is a physics-aware symbolic regression engine which proposes compact mathematical equations using deep reinforcement learning with a dimensional-analysis grammar and imposable constraints. The second, NestyNet, assembles networks with analytic derivatives and trains them using second-order methods, yielding fast, high accuracy fits to datasets, solvers for ODEs/PDEs, action-angle transformations, Gaussian-mixture inference, and dynamical modelling, with exact gradients and Hessians throughout. I will demonstrate how symbolic search coupled with accurate derivatives and with PDE constraints can rediscover analytic solutions from textbook physics. This approach is a practical route toward explainable, robust models for the forthcoming data deluge-aimed less at "automating Kepler" than at accelerating analysis while keeping physical insight. To arrange a visit with the speaker during the visit, please contact their host: Morgan Fouesnau

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As the largest gravitationally bound objects in the universe, forming relatively recently in cosmic time, the space and mass distributions of galaxy clusters are sensitive probes of the underlying cosmology - driven largely by dark matter and dark energy - in which clusters grow. Obtaining accurate galaxy cluster masses is therefore crucial. However, the primary method for deriving cluster masses relies on the hot X-ray emitting gas between galaxies, which results in masses about 40% lower than expected in the currently preferred cosmological model. I will discuss various factors contributing to this discrepancy, including modeling assumptions and instrumental calibration uncertainties - none of which can satisfactorily account for the entirety of the bias, including perhaps turbulent pressure support of the gas, based on recent results from the XRISM Observatory - and the implications for cluster physics and/or our understanding of cosmology.

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To arrange a visit with the speaker during the visit, please contact their host: Genevieve Parmentier

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The chemical composition of galaxies provides valuable insight into their formation and evolutionary pathways across cosmic time. In this talk, I will focus on gas-phase oxygen and nitrogen abundances as key tracers of chemical enrichment, and on the nitrogen-to-oxygen (N/O) ratio as a diagnostic of nucleosynthesis and star formation histories. These quantities can be derived from nebular emission lines, either through direct temperature-sensitive methods or strong-line calibrations. I will discuss how these approaches are applied in large spectroscopic surveys such as CALIFA, MaNGA, and DESI, and highlight the relations between oxygen and nitrogen abundances and global galaxy properties. Finally, I will show how spatially and statistically resolved abundance patterns provide constraints on the role of gas inflows, outflows, and feedback in shaping the baryon cycle of galaxies.

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The formation history of giant planets inside and outside the solar system remains unknown. I will present a new path for giant planet formation where runaway gas accretion is initiated only at a mass of ~100 M_Earth. This suggests that the transition to a gas giant planet, a planet whose composition is dominated by hydrogen and helium, occurs at ~Saturn’s mass. Delaying runaway accretion to later times (a few Myr) and higher masses is likely to be a result of an intermediate stage of efficient heavy-element accretion that provides sufficient energy to hinder rapid gas accretion. This implies that Saturn has never reached runaway gas accretion, and that it is a "failed giant planet". The transition to a gas giant planet above Saturn's mass naturally explains the differences between the bulk metallicities and internal structures of Jupiter and Saturn, and the characteristics of Uranus and Neptune. In terms of giant exoplanets, delaying runaway gas accretion to planets beyond Saturn's mass explains the transitions in the mass-radius (M-R) relations of observed exoplanets and the high metallicity of intermediate-mass exoplanets. To arrange a visit with the speaker during the visit, please contact their host: Saskia Hekker

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The escape of Lyman-alpha photons from galaxies is determined by the complex interplay of gas distribution, scattering processes, and radiative transfer. Lyman-alpha is widely used as a tracer of gas and ionizing radiation, yet its resonant nature makes the interpretation of observed spectra challenging and still not fully understood. To address this, we performed Monte Carlo simulations of Lyman-alpha propagation through anisotropic media, exploring slab geometries with diverse channel morphologies, outflows, dust content, and column density distributions - including lognormal profiles characteristic of the ISM. Since the interstellar medium is known to be porous, we are particularly interested in understanding how Lyman-alpha photons interact with and escape through low-density channels. Our results show that Lyman-alpha photons rarely escape solely through low-density channels; instead, they undergo extensive scattering, which enhances overall transmission. This indicates that Lyman-alpha emission is sensitive to the global distribution of neutral hydrogen rather than being confined to paths of least resistance. I will present the implications of these results for interpreting Lyman-alpha observables, focusing on how low- density channels influence the escape of Lyman-alpha photons and the information they provide about the interstellar medium.

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APEx Signature Speaker

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To arrange a visit with the speaker during the visit, please contact their host: Brian Reville

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Observations with the JWST have now revealed that massive star clusters at high redshifts formed with high mean stellar surface densities. These clusters are thought to represent the progenitors of globular clusters (GCs) that are nowadays found in practically all galaxies but whose origins remain poorly understood. In the dense, low-metallicity conditions of hierarchical proto-GC formation, massive stars and black holes have been theorized to grow in mass due to collisions while the star-forming gas is being self-enriched by stellar winds of massive stars. In this talk I will explore the formation of star clusters in such extreme environments by presenting the results of star-by-star hydrodynamical simulations of low-metallicity dwarf galaxy starbursts. The simulations account for a multiphase interstellar medium, stellar radiation, winds and supernovae, and accurate small-scale gravitational dynamics near massive stars. The latest simulation includes prescriptions for the collisional growth of very massive stars and tidal disruption events by stellar black holes. I will discuss the galactic population of star clusters, concentrating on the internal structure and chemical contents of massive clusters where the hierarchical formation history leaves a kinematic imprint in their stellar populations.

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Stellar feedback shapes the multi-phase interstellar medium in galaxies and the stellar initial mass function. Moreover, feedback impacts the large-scale evolution of galaxies by regulating star formation and by driving galactic fountain flows and outflows. Using modern high-performance computing simulations, we can study the relative importance of stellar winds, radiation, and supernovae in shaping the multi-phase interstellar medium. Using these simulations, we find that pre-supernova feedback is highly relevant for regulating star formation. In particular, the ionizing radiation of massive stars is dominating over the impact of non-ionizing radiation or stellar winds. On the other hand, supernovae drive hot bubbles and super-bubbles with substantially higher pressure than the typical midplane pressure of a disk galaxy, thereby pushing gas out into the circum-galactic medium. Additionally, cosmic rays, which are in this context most importantly accelerated by supernova shocks, help to sustain the galactic outflow via a vertical cosmic ray pressure gradient. In this talk, I will give an overview of the importance of stellar feedback for the evolution of galaxies, which we study using numerical simulations. To arrange a visit with the speaker during the visit, please contact their host: Cormac Larkin

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The High Energy Stereoscopic System (H.E.S.S.) is an array of imaging atmospheric Cherenkov telescopes (IACTs) that has been used to observe the sky in TeV gamma rays since 2004. Thanks to its unique location in the Southern Hemisphere and several upgrades to the system, the experiment continues to enable cutting-edge astrophysics despite its age. In my talk I will showcase the astrophysics that can be probed with IACTs, focusing on recent scientific highlights from H.E.S.S.

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The vast majority of known exoplanets orbits their host stars at quite close distances, compared to what we are used to from our own solar system. This proximity lets exoplanets and their host stars interact in significant ways through gravitational, magnetic, and radiation fields. The various flavours of those so-called Star-Planet Interactions manifest themselves as rotational and magnetic anomalies of the host stars' behavior, as well as in some cases as dramatic atmospheric evaporation of the exoplanets. Planet-induced alterations of stellar behaviour are hard to detect observationally, since cool stars display a number of stochastic magnetic phenomena of their own, which need to be distinguished from planetary effects. However, cleverly designed experiments and a wealth of space-based and ground-based data have allowed the field to make a lot of progress over the past decade. I will show exciting new results for observations of Star-Planet Interactions in this talk, and give an outlook on the possibilities for the field in the near future. To arrange a visit with the speaker during the visit, please contact their host: Joachim Wambsganss

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Understanding galaxies and their evolution over time is critical for interpreting the stellar history and chemical enrichment of the Universe. Galaxies that lie at different distances help us fill in different pieces of the puzzle - from large statistical samples of coarsely-resolved systems all the way down to our own Milky Way, which gives us a unique, up-close picture of galaxy structure and fundamental stellar physics. I will describe the Milky Way and Local Group in the context of galactic studies and highlight recent work that explores how stars and gas shape the evolution of galaxies on a wide variety of scales. With numerous new surveys in progress and on the horizon, including Gaia, the next generation of SDSS, and the Roman Space Telescope, these topics promise to be exciting avenues of research for many years to come.

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The discovery of billion-solar-mass black holes within the first Gigayear of cosmic history presents an intriguing puzzle: how did supermassive black holes (SMBHs) grow so rapidly in such a short amount of cosmic time? In this talk, I will introduce new approaches to probing the early growth of SMBHs. First, I will present the first measurement of the clustering strength of luminous quasars and their surrounding galaxies at z>6 using recent observations from the James Webb Space Telescope. These measurements allow us to infer the properties of the quasars’ host dark matter halos and their duty cycles, offering new insight into the environments that foster SMBH growth. I will then highlight new results from deep spectroscopic observations of background galaxies behind a luminous high-redshift quasar, which allow us to tomographically map the quasar’s ionized bubble, constraining the obscured fraction of quasars, their emission geometry, and the timescales of SMBH growth. To arrange a visit with the speaker during the visit, please contact their host: Nadine Neumayer

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Cold molecular gas is a key component in galaxy evolution, as it forms stars, bears feedbacks, and feeds supermassive blackholes. Interferometric observations of the Atacama Large Millimeter/submillimeter Array (ALMA) have remarkably advanced this field in the past decade. For spiral galaxies, a larger sample with higher physical resolution than before is systematically surveyed. I will discuss the molecular gas morphology and kinematics in three megamaser (Seyfert-II) galaxies at resolutions of around 100 pc. We found prevalent irregularities, potentially related to active galactic nucleus feedback and supermassive black-hole feeding. For early-type galaxies, it is now feasible to spatially resolve giant molecular clouds (GMCs). I will talk about GMCs at 15-pc resolution of the lenticular galaxy, NGC1387. Their dynamical states (Larson relations, virial parameters, etc.) are surprisingly similar to those in spiral galaxies. For our own Milky Way centre, a new ALMA large programme has mapped the central molecular zone (CMZ) at unprecedented spatial (sub-pc) and spectral (0.2 km/s) resolutions. Here, we found evidence of galactic shear effects and magnetic fields driving gas structure morphologies. I will conclude by summarising the physical drivers of molecular gas properties at different scales and in different environments.

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Galaxy clusters, representing the peaks in the cosmic density field, serve as an independent and powerful tool for investigating the evolution of cosmic structures. The strategic identification of these clusters through multi-wavelength surveys is essential for advancing our understanding of gravitational theory, general relativity, and cosmological models. Launched in 2019 aboard the Spectrum-RG mission, eROSITA marked a major milestone in astronomy by enabling the construction of the largest pure sample of galaxy clusters and groups detected through their hot intra-cluster medium in the X-ray band. In this talk, I will present results from my group’s work on deriving cosmological constraints from the evolution of the cluster mass function, combining eROSITA data with optical surveys such as DESI Legacy, DES, HSC, and KIDS. These parameters are constrained at a percentage level through the evolution of the cluster mass function, representing a significant leap forward. Beyond cosmology, a central focus of my research is on AGN feedback and its role in shaping galaxy and structure formation. Leveraging the statistical power of the eROSITA sample, we have detected warm baryons within cosmic filaments and cluster outskirts, offering a first glimpse of baryons in the faint, diffuse cosmic web. To arrange a visit with the speaker during the visit, please contact their host: Matteo Maturi

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KoCo Signature Speaker (GC)

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