Interaction of Cosmic Rays with Cold Clouds in Galactic Halos

TL;DR

This paper explores how cosmic rays influence cold clouds in galactic halos, affecting their dynamics, structure, and observable properties, with implications for galactic wind mechanisms and cloud-halo interactions.

Contribution

It introduces a novel analysis of cosmic ray effects on cold clouds, highlighting the role of CR pressure gradients and heating in cloud dynamics and observable ionic ratios.

Findings

CRs create a pressure gradient that pushes clouds upward.

CR heating broadens cloud-halo interfaces, affecting ionic abundances.

CR-mediated fronts match observed ionic ratios better than conduction fronts.

Abstract

We investigate the effects of cosmic ray (CR) dynamics on cold, dense clouds embedded in a hot, tenuous galactic halo. If the magnetic field does not increase too much inside the cloud, the local reduction in Alfv\'en speed imposes a bottleneck on CRs streaming out from the star-forming galactic disk. The bottleneck flattens the upstream CR gradient in the hot gas, implying that multi-phase structure could have global effects on CR driven winds. A large CR pressure gradient can also develop on the outward-facing edge of the cloud. This pressure gradient has two independent effects. The CRs push the cloud upward, imparting it with momentum. On smaller scales, the CRs pressurize cold gas in the fronts, reducing its density, consistent with the low densities of cold gas inferred in recent COS observations of local $L_{*}$ galaxies. They also heat the material at the cloud edge, broadening the cloud-halo interface and causing an observable change in interface ionic abundances. Due to the much weaker temperature dependence of cosmic ray heating relative to thermal conductive heating, CR mediated fronts have a higher ratio of low to high ions compared to conduction fronts, in better agreement with observations. We investigate these effects separately using 1D simulations and analytic techniques.