The Role of Cosmic-ray Transport in Shaping the Simulated Circumgalactic Medium

Abstract

Abstract The majority of galactic baryons resides 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 of them 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 nonthermal 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 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 farther 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 motivates the need for detailed parameter studies of CR transport.