The Impact of Cosmic Rays on Thermal and Hydrostatic Stability in Galactic Halos

Abstract: We investigate how cosmic rays (CRs) affect thermal and hydrostatic stability of circumgalactic (CGM) gas, in simulations with both CR streaming and diffusion. Local thermal instability can be suppressed by CR-driven entropy mode propagation, in accordance with previous analytic work. However, there is only a narrow parameter regime where this operates, before CRs overheat the background gas. As mass dropout from thermal instability causes the background density and hence plasma $\beta \equiv P_g/P_B$ to fall, the CGM becomes globally unstable. At the cool disk to hot halo interface, a sharp drop in density boosts Alfven speeds and CR gradients, driving a transition from diffusive to streaming transport. CR forces and heating strengthen, while countervailing gravitational forces and radiative cooling weaken, resulting in a loss of both hydrostatic and thermal equilibrium. In lower $\beta$ halos, CR heating drives a hot, single-phase diffuse wind with velocities $v \propto (t_\mathrm{heat}/t_\mathrm{ff})^{-1}$, which exceeds the escape velocity when $t_\mathrm{heat}/t_\mathrm{ff} \lesssim 0.4$. In higher $\beta$ halos, CR forces drive multi-phase winds with cool, dense fountain flows and significant turbulence. These flows are CR dominated due to "trapping" of CRs by weak transverse B-fields, and have the highest mass loading factors. Thus, local thermal instability can result in winds or fountain flows where either the heat or momentum input of CRs dominates.