Cosmic-Ray Transport in Varying Galactic Environments

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 et al. to postprocess 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 Alfvén speed, and diffusion relative to the Alfvén waves, with the diffusion coefficient set by the balance between streaming-driven Alfvén 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 postprocessed simulations to make predictions regarding the 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 extraplanar 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.