Interaction of Cosmic Rays with Cold Clouds in Galactic Halos

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 Alfvén 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 $L_{\ast }$ 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.

16 pages, 13 figures

References in corpus (7)

• The effect of photo-ionization on the cooling rates of enriched, astrophysical plasmas
• Time-Dependent Ionization in Radiatively Cooling Gas
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Cited by in corpus (7)

• Cosmic ray-driven galactic winds: streaming or diffusion?
• Simulations of cosmic ray propagation
• Cosmic-Ray Transport in Varying Galactic Environments
• The Role of Pressure Anisotropy in Cosmic Ray Hydrodynamics
• Dynamics of the Parker-Jeans instability of gaseous disks including the effect of cosmic rays
• Outflows in the presence of cosmic rays and waves
• Outflows in the presence of cosmic rays and waves with cooling