The Role of Cosmic Ray Transport in Shaping the Simulated Circumgalactic Medium

TL;DR

This study investigates how different cosmic ray transport mechanisms influence the structure and properties of the circumgalactic medium in galaxy simulations, highlighting the robustness of streaming over diffusion methods.

Contribution

It introduces and compares anisotropic cosmic ray transport models in galaxy simulations, demonstrating the impact on CGM structure and advocating for the use of streaming as a more reliable method.

Findings

CR streaming supports a multiphase, extended CGM with cool gas up to 200 kpc.

Diffusion methods produce a more uniform, warm CGM with fewer low-ion column densities.

Streaming shows less sensitivity to parameter variations, indicating greater robustness.

Abstract

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.