Cosmic Ray Transport, Energy Loss, and Influence in the Multiphase Interstellar Medium

TL;DR

This study uses high-resolution simulations to explore how cosmic rays propagate and influence multiphase interstellar media, revealing the importance of cloud interfaces and ionization effects on cosmic ray transport and feedback.

Contribution

It introduces detailed simulations of cosmic ray interactions with partially neutral clouds, highlighting the role of ionization-dependent transport and cloud interfaces in cosmic ray dynamics.

Findings

Cloud interfaces are key regions for cosmic ray pressure gradients.

Ionization-dependent transport slightly reduces cloud acceleration.

Gamma-ray emission maps are affected by transport mechanisms.

Abstract

The bulk propagation speed of GeV-energy cosmic rays is limited by frequent scattering off hydromagnetic waves. Most galaxy evolution simulations that account for this confinement assume the gas is fully ionized and cosmic rays are well-coupled to Alfv\'en waves; however, multiphase density inhomogeneities, frequently under-resolved in galaxy evolution simulations, induce cosmic ray collisions and ionization-dependent transport driven by cosmic ray decoupling and elevated streaming speeds in partially neutral gas. How do cosmic rays navigate and influence such a medium, and can we constrain this transport with observations? In this paper, we simulate cosmic ray fronts impinging upon idealized, partially neutral clouds and lognormally-distributed clumps, with and without ionization-dependent transport. With these high-resolution simulations, we identify cloud interfaces as crucial regions where cosmic ray fronts can develop a stair-step pressure gradient sufficient to collisionlessly generate waves, overcome ion-neutral damping, and exert a force on the cloud. We find that the acceleration of cold clouds is hindered by only a factor of a few when ionization-dependent transport is included, with additional dependencies on magnetic field strength and cloud dimensionality. We also probe how cosmic rays sample the background gas and quantify collisional losses. Hadronic gamma-ray emission maps are qualitatively different when ionization-dependent transport is included, but the overall luminosity varies by only a small factor, as the short cosmic ray residence times in cold clouds are offset by the higher densities that cosmic rays sample.