The Institut für Astrophysik Kolloquium occurs weekly and provides a chance for researchers from all over the world to present exciting new results and interact with members of our department during their visit.


The colloquia take place on Mondays at 15:00 in the Hörsaal of the institute building in Sternwartepark, and are also streamed online at this Zoom link.


If you have visitors or collaborators whom you think would make great colloquium speakers, please contact any of the organizers (Ryan Leaman, Glenn van de Ven, Sudeshna Boro Saikia, Florian List, Alvaro Hacar, Núria Miret Roig



  • Monday June 17: Sascha Quanz

    Abstract: TBD

  • Monday June 17: Stefanie Walch-Gassner - "The energetics of molecular clouds: reassessing the role of gravity"

    Abstract: The intricate dynamics of molecular clouds, pivotal to the formation of stars, have been a subject of our ongoing investigation. In the SILCC-Zoom simulation suite, we investigate the formation and evolution of molecular clouds within their galactic, multi-phase ISM environment. The adaptive mesh refinement (AMR) magneto-hydrodynamics simulations include a chemical network and radiative transfer. The clouds are divided into coherent and hierarchical substructures using dendrograms. A detailed virial analysis of the identified substructures unravels their energetics. We can show that substructures that mostly consist of atomic, rather than molecular gas, are always unbound. Only denser, molecular structures become "bound", but often they seem to be stabilized by ram pressure or thermal pressure rather than truly bound by self-gravity. Only a few structures are clearly gravitationally bound. Our results put the role of gravity to the test and have important implications for the low star formation rate and efficiency in molecular clouds.

  • Monday June 10: Hannah Diamond-Lowe - " Hot rocks and their host M dwarfs"

    Abstract: Exoplanets outnumber stars in the Milky Way, and small worlds with radii less than 4x Earth's radius are the most common. Despite being all around us, terrestrial exoplanets are the most difficult planetary type to study because they produce small signals in techniques we use to study exoplanets and their atmospheres, such as transit spectroscopy. To maximize detection and characterization signals we take advantage of "the M dwarf opportunity," whereby the relative radii and masses between small planets and small stars boosts the transit and radial velocity signals we are looking for. Today, JWST's infrared capabilities, pointing stability, and 25 square-meter collecting area are giving us unprecedented access to rocky worlds orbiting our closest M dwarf neighbors. However M dwarf host stars are more variable than expected, and can imprint stellar molecular features on planetary transit spectra. In this talk I will present the state of the field of terrestrial exoplanet science in the JWST era, including the latest results, toughest challenges, and brightest prospects for discovery.

  • Monday June 3rd: Ravit Helled - "What makes a planet 'giant'?"

    AbstractThe 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 that its composition is dominated in 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. 

  • Monday May 27: Jia Liu - "Cosmology with Massive Neutrinos"

    Abstract:  Ghostly neutrino particles continue to bring surprises to fundamental physics, from their existence to the phenomenon of neutrino oscillation, which implies their nonzero masses. Their exact masses, among the most curious unknowns beyond the Standard Model of particle physics, can soon be probed by the joint analysis of ongoing and upcoming cosmological surveys including Rubin LSST, Euclid, Roman, DESI, PFS, Simons Observatory, CMB-S4, and LiteBRID. In this talk, I will discuss ongoing works studying the effects of massive neutrinos and will draw a roadmap towards discovering the neutrino mass over the next decade.

  • Monday May 13: Julia Walter-Roszjár - "What is trickling down on us? A foray through meteoritics"

    Abstract: The research fields of meteorites and cosmochemistry are highly divers and interdisciplinary. The presentation will provide an overview about meteorite types and classes, their source bodies and regions, diverse mineralogy and characteristics of some selected meteorite types and related research projects. What can we find on Earth? What is the difference to terrestrial rocks? And what is the connection to astrophysics? Some aspects of the so-called micrometeorites and a conjunct scientific-artistic project to be installed at the Heldenplatz, Vienna will be highlighted in addition.

  • Monday May 6: Cora Uhlemann - "Cosmology from the dark universe to galaxy surveys"


    In this talk, I will take you on a journey through our Universe across the largest scales and the longest times. We will look at the dark side of our Universe, dark matter and dark energy, that constitute 95% of today’s total energy budget. We will see a cosmic web of structure emerge from the tug of war between the gravitational pull of dark matter and the expansion driven by dark energy. As galaxies form within the skeleton of dark matter, we can use large galaxy surveys as a laboratory for fundamental physics. From this year, the Euclid space telescope and Rubin Legacy Survey of Space and Time will map the distribution of billions of galaxies across most of the sky and over 10 billion years of cosmic history. By identifying locations and shapes of galaxies, we can connect dark matter to galaxies and probe the rise of dark energy that determines the fate of our Universe. Analysing billions of galaxies across huge volumes is a big data challenge involving nonlinear physics, commonly tackled by only considering simple averages. I will describe how we can squeeze out more information by probing the cosmic large-scale structure beyond the average of standard forward models and statistical analyses.

  • Monday April 22: Jorryt Matthee - "How galaxies and supermassive black holes changed the early Universe: insights after 1 year of JWST"

    Abstract: Identifying the first galaxies that emerged in the early Universe, characterising the properties of their stars and explaining the presence of supermassive black holes in galaxies at the highest redshifts are among the key goals of observational astrophysics. With its massive increase in imaging and spectroscopic capabilities in the infrared, the James Webb Space Telescope (JWST) is making significant progress in all these goals. In my talk, I will review lessons learned on galaxies in the early Universe after 1 year of JWST data. I will focus on the first spectroscopic census of emission-line galaxies in the EIGER survey of distant quasar fields, and the ALT survey of a powerful lensing cluster. These data informs us about the typical properties of stars, the chemical enrichment of the Universe, the halo masses and environments of the rarest quasars and show direct evidence that galaxies reionized the Universe. I will also discuss the surprising discovery of a population of faint active galactic nucleii (AGN) that may help us understand how the first supermassive black holes formed.

  • Monday April 15: Roser Pello - "MOSAIC, a unique instrument for the ELT"

    Abstract: MOSAIC is the future multi-object fibre spectrograph for the ESO 39m Extremely Large Telescope (ELT), working in both the Visible and NIR domains. It is a first generation instrument, with unique capabilities in terms of multiplex, wavelength coverage and spectral resolution. While ELT first light instruments will be using only the central part of the ELT focal plane, MOSAIC is designed to cover the largest possible area (~40 arcmin²) thanks to an original stepped and tiled focal plane design which allows parallel observations in different modes. MOSAIC is optimized to achieve the best possible signal-to-noise ratio on the faintest sources, from our Galaxy to the epoch of the reionization. In this respect, it will be a privileged survey machine for the followup of sources being discovered by current and future major facilities, such as JWST, Euclid, ROMAN or SKA. I will review the main expected contributions of MOSAIC to extragalactic science, focusing on the study of the first galaxies formed in the Universe.

  • Monday March 25: Giuseppina Battaglia - "The properties of satellite galaxies in the context of their orbits around the Milky Way"
    Abstract: The Local Group hosts about 100 dwarf galaxies, i.e. galaxies with stellar masses at least one order of magnitude lower than that of the Milky Way. As the smallest galaxies and the most dark matter dominated ones that we can observe in detail, Local Group dwarf galaxies are widely considered as precious systems to shed light on the processes that drive galaxy formation and evolution at small halo masses. Nonetheless, the majority of Local Group dwarf galaxies are found in the surrounding of a much larger system, i.e. the Milky Way or M31; this makes the knowledge of the orbits around the host a crucial piece of information for investigating the role of environment in shaping the dwarf galaxies' properties. In this talk I will present results from recent works, in which the exquisite astrometric data from the (early) third release of data from the ESA Gaia mission were used to provide probabilities of membership and systemic proper motions for Local Group dwarf galaxies. I will then discuss the orbital properties determined for the sub-set of about 50 dwarf galaxies found within 500 kpc from the Milky Way in the context of dwarf galaxies's group infall, tidal disruption from the Milky Way, and possible impact on the dwarfs star formation histories, as well as discuss some of the possibilities that clean samples of member stars from Gaia data are offering. Finally, I will briefly introduce the plans for the upcoming Galactic Archaelogy surveys with the WEAVE instrument, mounted at the prime focus of the William Herschel telescope.
  • Monday March 18: Jarita Holbrook - "How Astrophysicists Discriminate"


    *** Note: this will take place at Lecture Hall 2, UZA II, Floor 1, 2A122 & via u:stream ***

    Special colloquium within the Faculty Public Lecture Series:

    "Academia and In/Equality"


    Abstract: Holbrook has been studying astrophysicists and astrophysics culture for over a decade, on top of having been educated as an astrophysicists before moving into the social sciences. In order to contextualize their experience in the field and to aid others in navigating the field, Holbrook studies astrophysicists practices of inclusion and exclusion. This presentation focuses on the myths of intellectual superiority, white entitlement, sexism and other forms of discrimination including anti-Black discrimination. Included are film clips of astrophysicists talking about these issues.

    Remote questions can be asked through the SRS system at this link:



  • Monday March 18: Laura Magrini - "From Gaia-ESO to the next generation of high-resolution spectrographs"

    *** Note: Special Time 13:15 ***

    AbstractGround based specttroscopy is adding dimensionality to the astrometric,  photometric and spectroscopic data of the Gaia mission. Precise radial velocities, atomospheric parameters and abundances are enlarging our view of the Galaxy. Ingreasing the spectral resolution is improving our precision in measuring these quantities, but also is increasing the number of elements we can measure, tracing the history of the Galaxy with their different nucleosynthesis. Starting from a brief overview of some results obtained with the high resolution part of the Gaia-ESO survey, I will present two new facilities for the next decades focussed on high resolution spectroscopy, HRMOS -  a new spectrograph to be proposed for VLT- and WST - a new telescope fully dedicated to spectroscopy both MOS and IFS. 

  • Monday March 11: Javier Oliveras - "Bayesian methods for the characterization of stellar systems in the solar neighborhood"


    Within the Bayesian paradigm,  the Bayesian hierarchical models offer a robust and practical solution to the two most difficult caveats of classical artificial intelligence methods: interpretability and uncertainty propagation.
    In this presentation, we will briefly review the formalism behind the Bayesian hierarchical models and some of the latest probabilistic programming frameworks used for model construction, inference, and criticism.
    We will see practical examples of Bayesian models for the characterization of stellar associations, star-forming regions and open clusters in the solar neighbourhood. These applications go from membership methodologies for the discovery of ultracool dwarfs, the identification of kinematic substructures, and the inference of population-level distribution such as spatial distribution, velocity dispersion, mass, age, and energy distributions.  
    Finally, we will see an example of a hybrid model that unifies a Bayesian hierarchical model with an artificial neural network, thus providing an interpretable and robust solution for the inference of stellar system ages through isochrone-fitting and Lithium depletion boundary techniques. These hybrid models are a powerful emergent tool that is worth exploring.

    HostNúria Miret Roig

  • Monday March 4th: Emily Hunt - "The star clusters of the Milky Way"

    Star clusters are 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 the work I conducted during my PhD, 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 different unsupervised machine learning algorithms for detecting clusters in Gaia's billion-star dataset. Next, I will present our recently published catalogue of over 7000 clusters, which represents the largest homogeneous unduplicated catalogue of star clusters to date, including cluster classifications and parameters calculated with approximate Bayesian neural networks.

    Finally, I will discuss soon-to-be-published work on how bound and unbound star clusters can be better defined observationally, using their masses and Jacobi radii. I will present the largest ever catalogue of cluster masses and discuss many fascinating new results that come from this work, including: the first ever measurement of the global cluster mass function with Gaia data; the different lives of bound and unbound clusters; and how most clusters in the galaxy appear to have the same initial mass function - but only after first correcting for selection effects.

  • Monday February 26: Francisco Nogueras Lara - "The Dark Heart of the Galaxy: A Tale of Stars and Dust"

    Abstract: The heart of the Milky Way is our nearest galactic nucleus and the only one where it is possible to resolve individual stars down to milli-parsec scales. Therefore, it is a unique template for understanding other galactic nuclei and their role in galaxy formation and evolution. The Galactic centre is also the most prolific star-forming environment in our Galaxy when averaged over volume, making it a perfect laboratory to understand star formation under extreme conditions, similar to those in starburst or high-redshift galaxies. However, high crowding and extinction hinder its study, and even its morphology and kinematics are not yet entirely clear. The recent publication of new photometric and proper motion catalogues has allowed us to gain new insights into its properties. In this talk, I will describe our recent results on its stellar population, formation scenario, and morphology.

    Host: Stefan Meingast

  • Monday October 2: Philippe Bourdin - "The European Solar Telescope Project"


    The EST project gives great opportunities for novel observations of the Sun at unprecedented and photon flux rates. This leads to better resolution and higher spectral capabilities when observing the lower atmospheric layers. We will discuss the current and future project phases, as well as the technical and scientific challenges.

    Host: Bodo Ziegler



  • Monday October 9: Stella Offner - "Our Lonely Sun: How Multiple Star Systems Form (or don't)"


    Most stars are born with one or more stellar companions. Observational advances over the last decade have enabled high-resolution, interferometric studies of forming multiple systems and statistical surveys of multiplicity in star-forming regions. These have yielded new insights into how such systems form and how multiplicity affects disk evolution and planetary architectures. In this talk, I will review recent observational discoveries of the youngest multiple systems. I will present the results of star cluster simulations modeling the formation and evolution of multiple systems, and I will discuss the role of dynamics and environment in setting stellar multiplicity.  Finally, I will highlight remaining numerical and observational challenges.

    Host: Núria Miret Roig

  • Monday October 16: Alyson Brooks - "Interpreting Newly Discovered Dwarf Galaxies"


    New observational surveys (e.g., Rubin's LSST, WALLABY) are enabling the discovery of hundreds of nearby dwarf galaxies.  Are we theoretically ready for what the surveys will find?  The past decade has seen tremendous progress in simulating realistic dwarf galaxies.  In this talk, I'll outline a campaign to simulate the largest suite of dwarf galaxies to date, in environments both near the Milky Way and further afield.  These high-resolution, cosmological simulations are probing dwarf galaxy formation from LMC-mass scales down into the regime of the ultra-faints for the first time.  I will highlight some of the first predictions at the low-mass edge of galaxy formation, and highlight the importance of realistic star formation when interpreting the dark matter content of dwarf galaxies.

    Host: Ryan Leaman

  • Monday October 23: Chirag Modi - "Scaling simulation-based inference for the next generation of cosmological surveys"

    Simulation-based inference (SBI) is a promising approach to leverage cosmological simulations and extract information from the non-Gaussian, non-linear scales that cannot be modeled analytically. However, scaling SBI to the volumes and resolutions probed by the next generation of galaxy clustering surveys can be computationally prohibitive.  This is exacerbated by the fact that if we do not use accurate high fidelity simulations, SBI is susceptible to model misspecification. I will begin by putting this in context with discussing the sensitivity of SBI on the various components of cosmological simulations: gravity model, halo-finder and the galaxy-halo distribution models (halo-occupation distribution, HOD). Then, to overcome this computational bottleneck, I will present a new framework for cosmological analysis called Hybrid simulation-based inference (HySBI). HySBI combines perturbative methods (PT) on large scales with conditional SBI on small scales, thus learning the small-scale likelihood for a wide range of statistics using only small-volume simulations and drastically reduces computational costs. As a proof-of-principle, I will show results of using HySBI to constrain cosmological parameters on dark matter density fields using both the power spectrum and wavelet coefficients, finding promising results that significantly outperform classical PT methods. Finally, I will discuss a roadmap for the next steps necessary to implement HySBI on actual survey data, including consideration of bias, systematics, and customized simulations.

    Host: Oliver Hahn

  • Monday October 30: Paola Testa - "Observational signatures of coronal heating mechanisms in solar active regions"

    The details of the physical mechanisms responsible for heating the solar outer atmosphere to millions of degrees are still poorly understood. 
    Recent high resolution observations of the solar atmosphere (e.g., with SDO, Hinode, IRIS, SST) provide new powerful diagnostics of the coronal heating mechanisms. I will discuss how these new observations have enabled significant advances in our understanding of coronal heating properties in active regions. 
    I will especially focus on how high spatial, spectral, and temporal resolution coordinated observations of the chromosphere, transition region and corona, when coupled with state-of-the-art (1D/3D) models, can provide tight constraints on heating properties.

    Host: Manuel Guedel

  • Monday November 6: John Tobin - "The cradles of star and planet formation: disks, multiplicity, and stellar masses of low to intermediate-mass protostars"


    The formation of disks and multiple star systems are integral parts of the star and planet formation process. Most stellar mass must be accreted through a disk, disks are the future sites of planet formation, and disks will also give rise to companion stars. Large surveys of protostars in the nearby star forming regions are being conducted (with resolutions as fine as 5 au) to characterize the disk radii, disk masses, disk structure, and the frequency of multiplicity throughout the protostellar phase. The measured multiplicity properties will be discussed, along links back to their formation mechanisms. Protostellar disks appear to be systematically more massive that the proto-planetary disks, but have substructure less frequently than proto-planetary disks. Protostar mass measurements are beginning to become available for many 10s of sources, enabling their measurements to be compared to predictions for mass functions. Thus, the combined results from surveys of protostars bring us closer to unlocking the physics driving various aspects of star and planet formation.

    Host: Alvaro Hacar

  • Monday November 13: Sergio Torres Flores - "The role of galaxy interactions in compact groups of galaxies"


    Compact groups of galaxies are associations of three to six galaxies, located in a very small volume, where the separation between them is of the order of the diameters of the galaxies. Given that, galaxy-galaxy interactions and mergers are a common phenomena in compact groups of galaxies, transforming these systems into ideal laboratories to study galaxy evolution. Indeed, these strong interaction events can transform the morphological, kinematical and physical properties of galaxies. In this context, in this talk I will present a morphological analysis developed for a sample of galaxies located in 340 compact groups. We found a peculiar galaxy population, which has a smaller effective radius than galaxies located in less dense environments, indicating that the environment is playing a relevant role in the evolution of galaxies in compact groups. In addition, I will present a MUSE analysis on the merging compact group HCG 31, where we found perturbed kinematics and perturbed metallicity distributions. Finally, I will present some recent results associated with a search for vertical perturbations in local galaxies, which are typically associated with galaxy-galaxy interactions events. Thanks to the use of 3D Fabry-perot data, we found vertical perturbations which are consistent with high resolution simulations.

    Host: Bodo Ziegler

  • Monday November 27: Scott Lucchini - "Properties of the Magellanic Corona Model for the formation of the Magellanic Stream"

    AbstractWhen viewing the Large and Small Magellanic Clouds (LMC, SMC) in HI 21-cm emission, we see much more than the galaxies themselves. The Magellanic Stream, a massive structure comprised of intertwined, multiphase, gaseous filaments, trails behind the Clouds covering over 200 degrees on the sky. While previous models of the Stream's formation have shown us that many of its properties can be reproduced through interactions between the Magellanic Clouds, two puzzles remain - the large amount of ionized gas associated with the Magellanic System, and the high mass of the LMC. To solve both these outstanding discrepancies simultaneously, we introduce the Magellanic Corona. This warm, ionized circumgalactic medium should surround the LMC as it interacts with the SMC and the Milky Way. Recent observations detecting the Magellanic Corona around the LMC today provide further evidence for a high LMC mass and this leads to strong implications for the history of the Magellanic Clouds. I will present simulations of the formation of the Magellanic Stream including the Magellanic Corona in which we are able to account for both the neutral and ionized components of the Stream while also reproducing the present-day positions and velocities of the LMC and the SMC in their motion around the Milky Way. Upon inclusion of the Magellanic Corona, we find a new family of orbital histories for the Clouds that result in the neutral Stream being significantly closer than previous models predicted. I will present these new models and discuss some of the many implications of this new paradigm for the Magellanic Stream.

    Host: Cameren Swiggum

  • Monday December 4: John Peacock - "Galaxy formation in other universes"


    The outstanding puzzle of fundamental cosmology is the cosmological constant: an effective vacuum density that is tiny by comparison with predictions based on quantum physics. In 1987, Steven Weinberg suggested that this puzzle might be resolved by observer selection: natural large values for the vacuum density would suppress cosmological structure formation, so there would be no star formation in galaxies, and hence no observers. I will describe recent attempts to investigate this idea in detail, taking hydrodynamical simulation codes that succeed in modelling galaxy formation in our observed universe, and applying them to cosmologies with much larger cosmological constants. I will also contrast the simulation results with the predictions of a simplified analytic model for cosmic star formation. Different approaches to this problem yield somewhat different answers, but are in qualitative agreement: raising the cosmological constant does reduce the global efficiency of star formation, meaning that such models are less likely to be experienced by observers. But this suppression is relatively slow, so that cosmological constants more than 10 times the observed value would still form stars efficiently. Thus our observed universe has a vacuum density that is small compared with the value that would be typical in Weinberg’s model, and the question is whether this could be a statistical fluctuation that is not too rare.

    Host: Oliver Hahn

  • Monday December 11: Claudia Paladini - "Stellar surfaces through the looking-glass"



    Stellar convection plays an important role in many astrophysical processes, including energy transport, pulsation, dynamos and winds on evolved stars. A direct characterization of convective structures in terms of size, contrast, and life-span is quite challenging because stars are still pretty far and convective patterns are small. Most of our knowledge about stellar convection comes from studying the Sun. On the surface of our star a couple of millions of convective cells are observed, each one with a size of about 2000 km.Following predictions dating back to the ’70, the surface of evolved stars (or a Sun at later evolutionary stage) is expected to be populated by only a few large convective cells several tens of thousand times the size of the solar ones. Such predictions were confirmed at the end of last decade  by direct observations of the stellar surface of the low mass Asymptotic Giant Branch Stars pi1 Gruis. More recently the models are being challenged when it comes to more massive objects like the Red Supergiants. In this talk I will review the recent results obtained using high angular resolution techniques to resolve the surface of stars, and I will discuss the various scenarios used to interpret the images. Are we really looking at convection? 

    Host: Josef Hron