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• 24.04.2025 - Thomas Flöss (Uni. Wien) & Rebekka Coles-Bieri (UZH Zurich)

  Thomas Flöss (Uni. Wien)
  
  Inflation as a Cosmological Collider
  
  The hot big bang is presumed to have been preceded by a brief phase of rapidly accelerated expansion, known as cosmic inflation. Besides explaining the observed homogeneity, isotropy and flatness of the universe, it also naturally generates tiny density fluctuations that serve as the seeds of structure formation. I will discuss how inflation could have acted as a cosmological particle collider, at an energy scale far beyond that of terrestrial particle collider experiments, providing a unique window into high-energy physics. I will go on to discuss some prospects for studying the cosmological collider using cosmic structures.
  
  Rebekka Coles-Bieri (UZH Zurich)
  
  ARRAKIHS Mission Overview
  
  ARRAKIHS (Analysis of Resolved Remnants of Accreted galaxies as a Key Instrument for Halo Surveys) is ESA’s second F-class mission, designed to explore the low surface brightness universe beyond the Local Group. Its core objective is to constrain the nature of dark matter and the role of baryonic physics by detecting and characterizing faint stellar structures—such as tidal streams, shells, and diffuse halos—around 75 Milky Way analogues. These features preserve the imprints of hierarchical galaxy assembly and serve as sensitive probes of the underlying gravitational potential.
  
  To achieve this, ARRAKIHS will conduct deep, simultaneous visible and infrared imaging from space, reaching unprecedented low surface brightness limits. The mission’s observational requirements—including the field of view and dithering strategy—are directly informed by a suite of simulations that predict the extent, frequency, and morphology of these faint structures.
  
  In this talk, I will present the scientific goals of the mission and the simulation efforts that support and motivate the survey design. I will also highlight the structure of the international consortium and outline the timeline toward launch in 2030.
• 11.04.2025 - Cameren Swiggum (Uni. Wien) & Francesca Bonanomi (Uni. Wien)

  Cameren Swiggum (Uni. Wien)

  Rewinding 70 million years of nearby Star Formation with Gaia

  Gaia DR3 has revolutionized our understanding of the solar neighborhood, providing precise astrometry and spectroscopy to reconstruct the 3D history of local star formation. In this talk, I will present our evolving view of how star clusters and associations have formed within one kiloparsec of the Sun. We can now visualize the evolution of past star-forming complexes, mapping when and where star formation occurred within them over tens of millions of years. By integrating the orbits of young clusters backward in time, we uncover clear formation patterns driven by stellar feedback on hundred-parsec scales. This process of sequential star formation suggests that many of today’s youngest star-forming regions—such as Sco-Cen, Taurus, and Vela—likely emerged as a consequence of more massive complexes that preceded them. High-resolution 3D interstellar dust maps further reveal that large cavities in the ISM, shaped by clustered supernovae, are often occupied by the young clusters that formed from these past complexes. Additionally, the bulk motions of these previous complexes are not random; rather, they are dynamically linked to well-known kinematic structures in the solar neighborhood, namely the Pleiades and Coma Berenices moving groups. This connection suggests that the Galaxy’s non-axisymmetric forces, such as spiral arms, have played a role in shaping both the formation and motion of these complexes. With Gaia DR4, its enhanced kinematic precision and potential to uncover more lower-mass clusters will refine our view of the recent star formation history while also extending our ability to trace local star formation further back in time and farther from the Sun. This will reveal deeper connections between past and present star-forming complexes and their role within the broader dynamics of the Milky Way.

  Francesca Bonanomi (Uni. Wien)

  The formation of fibers in molecular clouds

  The presence of fibers in N2H+(1-0) has been detected above ~10^5 cm^-3 (~10^22 cm^-2) in several star-forming (SF) regions. These fibers are the velocity-coherent substructures of pc-scale filaments characterized by subsonic motion. The origin of these fibers is still under debate. In this work, we investigate how the diffuse and turbulent gas material connects to those dense fibers, charachterizing the environment where fibers formed. HNC(1-0), detected down to 5x10^21 cm^-2, becomes a key tool to probe this diffuse gas. We investigated the HNC(1-0) emission in a sample of 5 SF regions in Orion, part of the EMERGE Early ALMA Survey, observed at ~2000 au resolution. We have characterized the environment surrounding the fibers investigating of the diffuse gas kinematics, as velocity dispersion and turbulence regime. By exploring those gas properties in both low and high- mass SF regions we aim to investigate the origin of fibers in different environments.
• 04.04.2025 - Anne Hutter (Cosmic Dawn Center)

  Anne Hutter (Cosmic Dawn Center)

  Exploring variable stellar initial mass functions as an explanation for JWST's ultraviolet-bright z>10 galaxies

  JWST has revealed an unexpectedly high abundance of ultraviolet-bright z>10 galaxies, sparking discussions about the nature of star formation and the interstellar medium in the early universe. Which physical processes shaped their appearance? Do high gas densities enable feedback-free star formation, or does the nature and distribution of dust play a role? Is their star formation highly stochastic, or do they host a higher fraction of massive stars? We have used the Astraeus semi-numerical galaxy evolution and reionisation model to explore how different parameterisations of a variable stellar initial mass function (IMF) shape galactic scaling relations, the ultraviolet luminosity functions at z=5-15, and reionisation. In this talk, I will briefly review the proposed explanations for early ultraviolet-bright galaxies before focusing on how different variable IMFs affect the galaxy population. In particular, I will discuss what qualitative IMF dependence on galaxy properties is needed to explain z>10 JWST observations.
• 28.03.2025 - Sylvia Ploeckinger (Uni. Wien) & Efrem Maconi (Uni. Wien)

  Sylvia Ploeckinger (Uni. Wien)

  A sneak peek into the COLIBRE project

  The COLIBRE project - a large simulation project studying galaxy formation and evolution - is being completed at the moment and will be presented to the general public late Spring this year. The flagship simulations include cosmological cubic volumes with side lengths of 400 Mpc and 200 Mpc resolved with a particle mass (dark matter and baryons) of 10^7 and 10^6 solar masses, respectively, as well as smaller volumes (25, 50, and in a second stage 100 Mpc) with a particle mass of 10^5 solar masses. In this preview I will introduce the COLIBRE project, discuss the biggest improvements from previous simulation projects, such as EAGLE and IllustrisTNG, and show the first results. As the COLIBRE collaboration is currently inviting collaborators to use the COLIBRE data for various science projects, I will also outline the process to use the COLIBRE data already now.

  Efrem Maconi (Uni. Wien)

  
  Synthetic Faraday sky observed within a Local Bubble-like cavity

  
  Faraday rotation describes the change in the linear polarization angle of radiation passing through a magnetized plasma. The magnitude of this effect depends on the line-of-sight magnetic field component and the thermal electron density traversed by radiation - both key quantities for characterizing the Milky Way’s magnetic field. However, as the Solar System is within the Local Bubble, the impact of local structures on the Faraday signal might be underestimated. In this talk, I will present the synthetic Faraday sky as observed from the center of a Local Bubble-like cavity, selected within a magnetohydrodynamic simulation. I will discuss the imprint of the bubble walls on the Faraday signal and explore its actual origin in detail. Additionally, I will show the Faraday sky derived from diffuse polarized synchrotron radiation by Galactic cosmic ray electrons. I will conclude by emphasizing the role of upcoming Milky Way simulations in interpreting future high-resolution Faraday rotation maps.
• 21.03.2025 - Francisco Aros (Uni. Vienna) & Sebastian Hutschenreuter (Uni. Vienna)

  Francisco Aros (Uni. Vienna)

  Insights from the degree of energy equipartition in globular clusters.

  Two-body relaxation drives the dynamical evolution of Globular Clusters (GCs). As stars interact inside GCs, they exchange kinetic energy, making the clusters develop different degrees of energy equipartition. As new observations with HST and JWST bring high-quality kinematic and photometric data of stars in GCs down the faint end of the main sequence and out to multiple half-light radii, studying the degree of energy equipartition in GCs has become a new window into understanding their dynamical processes and evolution. In this talk, I will discuss how the observed degree of energy equipartition in GCs could provide insight into the presence of stellar-mass black holes and the mixing process in clusters with multiple populations.

  Sebastian Hutschenreuter (Uni. Vienna)

  Inferring the structure of the Galactic Magnetic Field

  The Galactic Magnetic Field (GMF) plays an important role in shaping the structure of the Milky Way by regulating fundamental processes such as star formation and the transport of charged particles in the interstellar medium (ISM). The GMF is shaped by the dynamic interplay of plasma and gravity, as well as stellar feedback. Understanding the GMF is not only fascinating in its own right but also essential for explaining extragalactic phenomena, particularly the Cosmic Microwave Background. Despite its significance, the three-dimensional structure of the GMF remains elusive. In my talk, I will showcase how we can map the GMF by employing modern statistical frameworks. Specifically, I will demonstrate the use of Faraday rotation to obtain direct estimates of the average GMF strength across the full sky, I will present a three-dimensional reconstruction of dust polarization source fields tracking the local GMF, and I will emphasize connections of these maps to the recent history of the local ISM.
• 14.03.2025 - Quentin Changeat (Uni. Groningen) & Oleg Savchenko (GRAPPA Institute)

  Quentin Changeat (Uni. Groningen)
  
  Interpretation of exoplanet atmospheres with space observatories.
  
  The characterization of exoplanets relies on precise observations from space-based telescopes, which provide crucial data on planetary atmospheres and their composition. Missions such as Hubble and JWST have already delivered high-quality spectroscopic data for hundreds of transiting and directly imaged exoplanets. In 2029, these observatories will be joined by Ariel, a dedicated ESA mission designed to study thousands of exoplanet atmospheres. Together, these missions will revolutionize the field, generating an unprecedented volume of high-quality data essential for identifying trends in exoplanet populations. In this talk, I will discuss how these observatories contribute to our understanding of exoplanetary systems, highlighting key discoveries as well as modern challenges in data reduction and atmospheric interpretation.
  
  Oleg Savchenko (GRAPPA Institute)
  
  Sequential simulation-based inference for cosmological initial conditions
  
  Knowledge of the primordial matter density field from which the present non-linear large-scale structure emerged is of fundamental importance for cosmology, as it contains an immense wealth of information about the physics, evolution, and initial conditions of the universe. Reconstructing this density field from galaxy survey data is a notoriously difficult task requiring advanced cosmological simulators and sophisticated statistical methods to explore a multi-million-dimensional parameter space. In this talk, I will discuss how simulation-based inference allows us to tackle this problem and obtain data-constrained realisations of the primordial dark matter density field in a simulation-efficient way for general non-differentiable simulators. In addition, I will describe how our method allows us to turn any initial conditions point estimator into a fast sampler, and our novel adaptive learning training strategy to simultaneously infer the initial conditions together with the cosmological parameters. (Mostly based on arxiv.org/abs/2502.03139)
• 21.02.2025 - Laura Scholz-Díaz (INAF - Osservatorio Astrofisico di Arcetri) & Graham Smith (Uni. Birmingham/Uni. Vienna)

  Laura Scholz-Díaz (INAF - Osservatorio Astrofisico di Arcetri)

  The impact of dark matter halos on the baryonic content of galaxies: Dynamical evidence from the CALIFA survey

  The interplay between the baryonic physics of galaxies and the assembly of dark matter halos is essential for understanding galaxy formation, but remains elusive to observations, which typically rely on indirect halo characterizations. In this talk, I will report direct observational evidence from the CALIFA survey showing that the baryonic properties of nearby galaxies -such as age, metallicity, star formation rate, morphology, stellar angular momentum- as well as the radial profiles and gradients of their stellar populations, are influenced by their host halos. Through detailed dynamical modeling of optical integral-field spectroscopic data, we found that for galaxies with similar stellar masses, these baryonic properties vary depending on their total enclosed mass (stars + dark matter). We demonstrate that total mass correlates with halo mass inferred from indirect methods, as well as in numerical simulations. Our findings indicate that dark matter halos play a key role in shaping the baryonic content of galaxies and suggest that the timing of halo formation could significantly impact observed galaxy properties.

  Graham Smith (Uni. Birmingham/Uni. Vienna)

  Adventures of a gravitational lenser in Vienna

  I will summarise some scientific highlights from my Winter Semester in Vienna. Of course gravitational lensing and Rubin/LSST will feature, but so too will hunting for extragalactic planets, and probing quantum physics with gravitational lensing in the Solar system. I will also summarise recent progress in Rubin/LSST on-sky commissioning, but sadly am not allowed to show any data.