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. (2021) 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 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 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.

Cosmic-ray diffusion and the multi-phase interstellar medium in a dwarf galaxy. I. Large-scale properties and $γ$-ray luminosities

“…We have tested three uniform diffusion coefficients, κ = 3 × 10 27 , 3 × 10 28 , and 10 29 cm 2 s −1 , which bracket the typical value of 3 × 10 28 (R CR /1 GV) 0.34 cm 2 s −1 inferred as a function of particle rigidity R CR in the Milky Way (Jóhannesson et al 2019). They also correspond to the range of values inferred by Armillotta et al (2022) in different interstellar environments.…”

“…CR stream at large speeds along field lines and the diffusion coefficient should again strongly depend on the ambient Alfvénic Mach number (Xu & Lazarian 2022). By post-processing 4-pc to 8-pc resolution simulations of the ISM to solve the CR transport equations, Armillotta et al (2022) find that the CR mean free path can vary by five orders of magnitude across the different gas states. Further complications arise in the H i and H 2 gas phases at parsec and sub-parsec scales.…”

“…In the low-density, hot ionised gas that fills the disc corona (aka halo), the Alfvén wave growth is mostly (but weakly) limited by non-linear Landau damping. CRs primarily are advected away by fast winds and/or by streaming at the large Alfvén speeds that prevail in the tenuous gas (Armillotta et al 2022). In the warm ionised medium that extends to 1 or 2 kpc about the plane, turbulent damping resulting from interactions between self-excited Alfvén waves and background MHD waves can significantly suppress the streaming instabilities (Farmer & Goldreich 2004;Xu & Lazarian 2022) even though it is often ignored in simulations.…”