Cosmic ray plasma physics and magnetic dynamos in galaxy formation

Understanding the physics of galaxy formation is an outstanding problem in modern astrophysics. Recent cosmological simulations have demonstrated that feedback by star formation, supernovae and active galactic nuclei appears to be critical in obtaining realistic disk galaxies and to slow down star formation to the small observed rates. However the particular physical processes underlying these feedback processes still remain elusive. In particular, many of these simulations neglected magnetic fields and relativistic particle populations (so-called cosmic rays). Those are known to provide a pressure support comparable to the thermal gas in our Galaxy and couple dynamically and thermally to the gas, which seriously questions their neglect. Early attempts to include magnetic fields and cosmic rays put the plasma kinetics of wave-particle interactions into the limelight for explaining the power of galactic winds and for regulating star formation in and cosmic accretion onto galaxies. After introducing the underlying physical concepts, I will present our recent efforts to model kinetic plasma physics in macroscopic hydrodynamical models of galaxy formation. In particular, I will explain how cosmic rays interact with and propagate through the magnetized plasma in the interstellar and circumgalactic media. I will elucidate the role of plasma instabilities in regulating the momentum transfer from cosmic rays to the background plasma and how we can observationally test these theoretical considerations using new high-sensitivity MeerKAT observations. I will then demonstrate that cosmic rays play a decisive role in the formation and evolution of spiral galaxies by providing feedback that regulates star formation and drives gas out in galactic winds. Comparing cosmic ray spectra of electrons and protons to observational data and studying the correlation of the far-infrared emission with the gamma-ray and radio emission from galaxies enables us to test the cosmic ray feedback and dynamo models for the growth of galactic magnetic fields. This argues that a complete understanding of galaxy formation necessarily includes plasma astrophysics.