Thermal instability of halo gas heated by streaming cosmic rays

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 k · B 0 are strongly affected by CR diffusion. When the streaming time is shorter than the CR diffusion time, thermally unstable modes (with k · B 0) are waves propagating at a speed ∝ the Alfvén speed. Halo gas in photoionization equilibrium is thermally stable independent of CR pressure, while gas in collisional ionization equilbrium 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 cool /t 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 ds eff /dz < 0, where s 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.

The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

“…In the streaming approximation of cosmic-ray transport, cosmic rays heat the gas at a rate proportional to the Alfvén velocity and cosmic-ray pressure gradient ( c in Equations (4) and (5)). We expect cosmic-ray heating to be significant in gas with low β and high η (Kempski & Quataert 2020). This heating process is also more efficient in simulations with larger t t cool ff , which are evolved long enough for cosmic-ray transport processes to become important.…”

“…With sufficiently high cosmic-ray pressures, gas cools at constant density (isochorically; Sharma et al 2010;Kempski & Quataert 2020).…”

“…There are several ways in which cosmic rays could alter the thermal instability in CGM gas. Nonthermal cosmic-ray pressure supports cold gas, enabling it to cool isochorically (Sharma et al 2010;Kempski & Quataert 2020), and may explain the inferred low densities of cold CGM gas. Cosmicray pressure also counteracts gravity and may prevent cold halo gas from accreting onto the galaxy (Hopkins et al 2020a).…”

“…In some regimes, this additional heating term can balance radiative cooling and has been shown to prevent gas from overcooling in simulations of galaxy clusters (Guo & Oh 2008;Sharma et al 2010;Jacob & Pfrommer 2017a, 2017b. Linear thermal stability analysis predicts that CGM gas (10 5 K<T< 10 7 K) is likely thermally unstable in the presence of streaming cosmic rays, but the predicted thermal instability growth rate depends on the invoked cosmic-ray transport mechanism (Kempski & Quataert 2020). However, linear analysis breaks down once thermal instability saturates, and there is a strong need for simulations to further understand the cold gas properties and subsequent evolution.…”

“…This could prevent cold gas clumps from precipitating after they condense out of the hot background medium. Additionally, if the cosmicray scale height is sufficiently large relative to the gas scale height,  H H 5 2 c g , gas becomes convectively unstable (Kempski & Quataert 2020). Our simulations are initialized with constant η, so H c /H g is always close to 1.…”