Cosmic Rays and the Circumgalactic Medium

All galaxies evolve embedded in a vast gaseous halo --- the circumgalactic medium (CGM) --- that dwarfs the mass and spatial extent of the stars in the galactic disk. Recent ultraviolet (UV) absorption studies reveal that the CGM has a rich, multiphase structure shaped by metal-rich galactic winds that expel the byproducts of stellar evolution and pristine inflows from the intergalactic medium. Galaxy evolution is intricately tied to the state of its CGM, as galaxies depend on a consistent supply of cool gas in order to form new stars. While current state-of-the-art simulations excel at reproducing galactic disk properties, most struggle to simultaneously reproduce the observed abundance, properties, and kinematic structure of multiphase CGM gas. In this talk, I will argue that the missing piece is likely a nonthermal component in the form of cosmic rays. I will first describe how cosmic rays can fundamentally alter the structure of the CGM and the implications for how we interpret existing observations. I will then discuss the large uncertainties in existing theoretical models of cosmic ray propagation and propose new observational tests and theoretical models for constraining these models.