Simulating Multiphase Galactic Halos: The Impact of Cosmic Rays Through an Observational Lens

Abstract

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 thesis, I argue that the missing piece is likely a nonthermal component in the form of cosmic rays. I implement cosmic-ray physics into the {\sc enzo} and {\sc ChaNGa} simulation codes and use a combination of idealized, isolated-galaxy, and cosmological-zoom-in simulations to demonstrate the role of cosmic rays in driving galactic outflows and shaping the multiphase CGM. Throughout this work, I demonstrate how uncertainties in existing cosmic-ray transport models can lead to qualitatively different CGM structures, and motivate the need for more robust models of cosmic-ray hydrodynamics. Additionally, I explore more direct techniques for comparing simulations against observations. Using synthetic spectroscopy, I demonstrate how cosmic rays can naturally explain the observed kinematic alignment of multiphase CGM gas. I then use synthetic observations to demonstrate the prevalence and observational signatures of cool and warm gas in the halos around the most massive galaxies, which are traditionally studied with X-ray emission from their hot, dense inner regions.