Cosmic-Ray Transport in Varying Galactic Environments

TL;DR

This study investigates how cosmic rays propagate in different galactic environments, revealing that higher star formation rates lead to more effective cosmic-ray transport and influence galactic wind dynamics.

Contribution

It introduces a detailed cosmic-ray transport model applied to high-resolution galaxy simulations, highlighting the impact of star formation activity on cosmic-ray behavior.

Findings

CR transport is more effective in high SFR environments due to hot outflows.

CR pressure ratios decrease with increasing SFR.

CRs may significantly influence galactic winds and fountain dynamics.

Abstract

We study the propagation of mildly-relativistic cosmic rays (CRs) in multiphase interstellar medium environments with conditions typical of nearby disk galaxies. We employ the techniques developed in Armillotta+21 to post-process three high-resolution TIGRESS magnetohydrodynamic simulations modeling local patches of star-forming galactic disks. Together, the three simulations cover a wide range of gas surface density, gravitational potential, and star formation rate (SFR). Our prescription for CR propagation includes the effects of advection by the background gas, streaming along the magnetic field at the local ion Alfv\'en speed, and diffusion relative to the Alfv\'en waves, with the diffusion coefficient set by the balance between streaming-driven Alfv\'en wave excitation and damping mediated by local gas properties. We find that the combined transport processes are more effective in environments with higher SFR. These environments are characterized by higher-velocity hot outflows (created by clustered supernovae) that rapidly advect CRs away from the galactic plane. As a consequence, the ratio of midplane CR pressure to midplane gas pressures decreases with increasing SFR. We also use the post-processed simulations to make predictions regarding potential dynamical impacts of CRs. The relatively flat CR pressure profiles near the midplane argue that they would not provide significant support against gravity for most of the ISM mass. However, the CR pressure gradients are larger than the other pressure gradients in the extra-planar region (|z|>0.5 kpc), suggesting that CRs may affect the dynamics of galactic fountains and/or winds. The degree of this impact is expected to increase in environments with lower SFR.