Thermal Instability of Halo Gas Heated by Streaming Cosmic Rays

TL;DR

This paper analyzes how streaming cosmic rays influence the thermal stability of halo gas, revealing conditions under which the gas remains stable or becomes unstable, affecting galaxy halo evolution.

Contribution

It provides a linear stability analysis considering CR streaming and diffusion, highlighting the impact on thermal modes and stability criteria in galaxy halo plasmas.

Findings

Halo gas in photoionization equilibrium is thermally stable regardless of CR pressure.

Gas in collisional ionization equilibrium can be thermally unstable under realistic conditions.

CR streaming can modify the stability thresholds and oscillation frequencies of halo gas.

Abstract

Heating of virialized gas by streaming cosmic rays (CRs) may be energetically important in galaxy halos, groups and clusters. We present a linear thermal stability analysis of plasmas heated by streaming CRs. We separately treat equilibria with and without background gradients, and with and without gravity. We include both CR streaming and diffusion along the magnetic-field direction. Thermal stability depends strongly on the ratio of CR pressure to gas pressure, which determines whether modes are isobaric or isochoric. Modes with $\mathbf{k \cdot B }\neq 0$ are strongly affected by CR diffusion. When the streaming time is shorter than the CR diffusion time, thermally unstable modes (with $\mathbf{k \cdot B }\neq 0$) are waves propagating at a speed $\propto$ the Alfv\'en speed. Halo gas in photoionization equilibrium is thermally stable independent of CR pressure, while gas in collisional ionization equilibrium is unstable for physically realistic parameters. In gravitationally stratified plasmas, the oscillation frequency of thermally overstable modes can be higher in the presence of CR streaming than the buoyancy/free-fall frequency. This may modify the critical $t_{\rm cool}/t_{\rm ff}$ at which multiphase gas is present. The criterion for convective instability of a stratified, CR-heated medium can be written in the familiar Schwarzschild form $d s_{\rm eff} / d z < 0$, where $s_{\rm eff}$ is an effective entropy involving the gas and CR pressures. We discuss the implications of our results for the thermal evolution and multiphase structure of galaxy halos, groups and clusters.