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

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.