Cosmic Ray Driven Outflows
The majority of galactic baryons reside outside of the galactic disk in the diffuse, multiphase gas known as the circumgalactic medium (CGM). Uncovering the physical processes that govern the galactic outflows and inflows that shape the complex multiphase structure of the CGM places rigid constraints to theories of galaxy formation and evolution. While state-of-the art simulations excel at reproducing galactic disk properties, many struggle to drive strong galactic winds or to match the observed multiphase structure of the CGM with thermal supernova feedback. It is likely that the missing key is a non-thermal component to stellar feedback. Cosmic rays may be that key.
Cosmic rays are charged particles (mostly protons) that are accelerated to relativistic speeds in extreme astrophysical shocks (like supernova explosions!). Although their velocities are so large, cosmic rays are scattered by magnetic fields and are confined within the galaxy for tens of millions of years. Cosmic rays interact with the thermal gas by providing pressure support. This added pressure can drive galactic outflows (see Figure 1) and significantly alter the temperature and ionization state of the gas in the CGM (see Figure 2).
Approximating cosmic ray transport along magnetic field lines in simulations is a challenge. Using a suite of isolated disk galaxy simulations, we showed that the state of the simulated CGM is sensitive to the choice of cosmic ray transport prescription. With this, we motivate the need for a robust prescription for cosmic ray transport and feedback.