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 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