High-Energy Astrophysics and Cosmology (HAC)

Cosmic rays consist of charged elementary particles that move incredibly fast, close to the speed of light. Just now, one century after the discovery of cosmic rays, we are starting to understand the astrophysical mechanisms underlying the acceleration of ordinary elementary particles to these enormous velocities: powerful shock waves driven by exploding stars or alternatively powered by mass accretion onto supermassive black holes. Are these cosmic rays just an astrophysical curiosity or do they play a decisive role in the evolution of cosmic structure?

This is the central question that we would like to answer with the research in our group “High-Energy Astrophysics and Cosmology”. First indications point to the possibility that cosmic rays could be responsible for driving powerful gaseous outflows during the formation of galaxies; thus explaining the low star formation observed and solving a major problem of galaxy formation.

Cosmic rays could even play a critical role during the evolution of clusters of galaxies, the largest bound objects in existence and an important touchstone in understanding the formation of structures in our Universe. However, the thermal history of clusters remain mysterious; some central regions should have long since cooled and collapsed, which constitutes the famous “cooling flow problem”. Instead, the most massive black holes in the Universe, which are situated at the center of these galaxy clusters, appear to heat the cooling gas at just the right rate. But how does this mechanism work? Again, elusive cosmic rays could come to the rescue and provide the necessary stable heating mechanism.

In our group, we are tackling these challenging problems with a combination of paper-and-pencil theory and an advanced simulation technique that employs a moving mesh as the underlying numerical structure; thus offering an unprecedented combination of accuracy, resolution and physical completeness. We complement our theoretical efforts with a focused observational program on the non-thermal emission of galaxies and clusters, taking advantage of new capabilities at radio to gamma-ray wavelengths.