Comparing subgrid models for cosmic ray diffusion in a magnetized isolated galaxy simulation

ABSTRACT Galactic cosmic rays (CRs) play a crucial role in galaxy formation and evolution by altering gas dynamics and chemistry across multiple scales. Typical numerical simulations of CR transport assume a constant diffusion coefficient for the entire galaxy, despite both numerical and theoretical studies showing that it can change by orders of magnitude depending on the phase of the interstellar medium. Only a few simulations exist that self-consistently calculate CR transport with diffusion, streaming, and advection by the background gas. In this study we explore three subgrid models for CR diffusion, based on popular theories of CR transport. We post-process an isolated star-forming magnetohydrodynamic (MHD) galactic disc simulated using the ramses code. The resulting diffusion coefficients depend solely on the subgrid turbulent kinetic energy and the MHD state variables of the plasma. We use these models to calculate coefficients for vertical transport. We find that they depend critically on the local magnetic field tilt angle. Across models, our resulting diffusion coefficients range from $10^{26}$ to $10^{31}~\rm cm^2s^{-1}$, and yield CR energy densities at the midplane from 1 to $100 ~\rm eV cm^{-3}$, suggesting varied degrees of backreaction on their environment. Using simple approximations, we show that the gamma-ray luminosity of the galaxy depends primarily on the gas surface density and the turbulent confinement of CRs by the galactic corona.