Multi-phase gas in and around galaxies: the impact of cosmic rays, magnetic fields and cooling processes

Christoph Pfrommer


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, 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. After introducing the underlying physical concepts, I will present our recent efforts to model magnetic fields and cosmic rays in galaxy formation. In particular, I will explain how cosmic rays interact with and propagate through the magnetized plasma in the interstellar and circumgalactic media 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. Finally, I will return to the classic problem of the interaction of a cold cloud in a hot wind: this is a rich problem that includes cooling physics, hydrodynamic instabilities and how these processes are altered in the presence of magnetic fields. I will show that our new insight into this problem has severe consequences for our understanding of observations of the multi-phase, magnetized plasma in and around galaxies.