The Role of Cosmic Ray Transport in Shaping the Simulated Circumgalactic
  Medium
  The majority of galactic baryons reside outside of the galactic disk in the
diffuse gas known as the circumgalactic medium (CGM). While state-of-the art
simulations excel at reproducing galactic disk properties, many struggle to
drive strong galactic winds or to match the observed ionization structure of
the CGM using only thermal supernova feedback. To remedy this, recent studies
have invoked non-thermal cosmic ray (CR) stellar feedback prescriptions.
However, numerical schemes of CR transport are still poorly constrained. We
explore how the choice of CR transport affects the multiphase structure of the
simulated CGM. We implement anisotropic CR physics in the astrophysical
simulation code, {\sc Enzo} and simulate a suite of isolated disk galaxies with
varying prescriptions for CR transport: isotropic diffusion, anisotropic
diffusion, and streaming. We find that all three transport mechanisms result in
strong, metal-rich outflows but differ in the temperature and ionization
structure of their CGM. Isotropic diffusion results in a spatially uniform,
warm CGM that underpredicts the column densities of low-ions. Anisotropic
diffusion develops a reservoir of cool gas that extends further from the
galactic center, but disperses rapidly with distance. CR streaming projects
cool gas out to radii of 200 kpc, supporting a truly multiphase medium. In
addition, we find that streaming is less sensitive to changes in constant
parameter values like the CR injection fraction, transport velocity, and
resolution than diffusion. We conclude that CR streaming is a more robust
implementation of CR transport and motivate the need for detailed parameter
studies of CR transport.
