Star Formation in crowds: Protostars and feedback in protoclusters

Main Colloquium
Prof. Adam Ginsburg
University of Florida, Gainesville, USA

Star formation is the defining process in the evolution of galaxies. Our understanding of star formation has primarily been informed by low-mass stars in nearby clouds, but these nearby regions do not reflect typical conditions over the history of the universe. The denser and more crowded regions that represent our own origins exist within our Galaxy, and ALMA allows us to explore these regions in ways previously impossible. My research group is working to count forming stars in high-mass protoclusters, with the ultimate aim of answering how the stellar initial mass function (IMF) forms from gas. I will discuss recent and ongoing efforts to count protostars and cores, measure their masses, and measure the gas they came from. These include the ALMA-IMF large program and comparable observations toward W49 and Sgr B2. I will highlight the recently-discovered lines of salt (NaCl and KCl) and CS masers as tools for measuring high-mass stars with disks. Finally, I will describe a planned small satellite mission, PASHION, that will map the Paschen Alpha line throughout the Galaxy.

The interplay between gas, dark gas, and cosmic rays in the local interstellar medium

Main Colloquium
Prof. Isabelle Grenier
Université Paris Diderot and CEA Saclay, France

The Milky Way stands as the most prominent source of GeV gamma rays in the sky as cosmic rays interact with the interstellar medium along their Galactic journey. The gamma-ray data from the Fermi Observatory show that GeV-TeV cosmic rays smoothly diffuse through cloud complexes and that they penetrate deeply into clouds. Their hadronic interactions expose all gas nuclei, independently of their thermal or chemical state. Nearby clouds have been probed in gamma rays to gauge the molecular mass present in the CO-bright regions and the large gas mass constituting the dark HI-H2 interface. I will review these results and show preliminary constraints on the dark-gas composition that point to a large abundance of diffuse H2 and other molecules (HCO+, C2H) with only little CO in this phase. In parallel, atomic clouds can serve as nuclear targets to study the local propagation of GeV to TeV cosmic rays, including their potential re-acceleration in the nearby Orion-Eridanus superbubble. While the cosmic rays appear to be rather uniformly distributed within a few hundred parsecs around the Sun, interesting deviations that I will discuss start to hint at a more complex relation between the particles and the magnetic-field structure of the local interstellar medium.

A brief history of black hole imaging

Main Colloquium
Dr. Jean-Pierre Luminet
Laboratoire d’Astrophysique de Marseille and Observatoire de Paris, France

After briefly commenting the 2020 Nobel Prize in Physics attributed to black hole studies, I'll be back to April 2019, when the Event Horizon Telescope Consortium provided the first telescopic image of the shadow of the supermassive black holes M87* embedded in its accretion structure, at a resolution scale comparable to the size of its event horizon. Well before this remarkable achievement made possible by VLBI radio astronomy, many researchers used the computer to reconstruct what a black hole surrounded by luminous material would look from close-up views. The images must experience extraordinary optical deformations due to the deflection of light rays produced by the strong curvature of the space-time in the vicinity. General relativity allows the calculation of such effects, both on a surrounding accretion disk and on the background star field. I'll give an exhaustive and illustrated review of the numerical work on black hole imaging done during the first thirty years of its history. I'll conclude with very recent and yet unpublished simulations about the internal structure of an idealised Kerr black hole.

Influence of early cluster dynamics on protoplanetary discs

Master Colloquium
Amith Govind
Forschungszentrum Jülich and Max-Planck-Institut für Radioastronomie


Weak gravitational lensing as a probe for cosmology

Special Colloquium
Dr. Tim Schrabback
Argelander-Institut für Astronomie, Bonn

The Universe appears to be dominated by dark matter (driving the growth of structure) and dark energy (driving the late-time accelerated expansion). Together, these invisible components contribute to 95% of the cosmic energy budget. Yet, their physical nature is still unclear. A powerful tool to learn more about them is weak gravitational lensing: The light bundles of background galaxies get distorted while passing through the gravitational potential of foreground matter concentrations. These distortions can be measured statistically, allowing us to reconstruct the foreground mass distribution, weigh cosmological objects, and probe the cosmological growth of structure. In this talk I will summarise several observational studies, in which we employed weak lensing measurements aiming to test and constrain the cosmological model. This includes stacked measurements of the weak lensing signal around large samples of foreground galaxies, where we search for the expected signature of dark matter halo flattening. Weak lensing is also an essential ingredient for cosmological investigations based on the number counts of galaxy clusters, since it allows us to robustly constrain their absolute mass scale. In this context I will present results based on our weak lensing follow-up campaign for clusters from the South Pole Telescope Sunyaev-Zel'dovich Survey. Finally, the statistics of the large-scale matter distribution can also be probed by correlating the distorted shapes of background galaxy pairs in high-resolution wide-area imaging surveys. As an example for such cosmological weak lensing measurements I will present some of our earlier results obtained using Hubble Space Telescope imaging, followed by an outlook into what will be possible with ESA's upcoming Euclid mission.