Virial shocks are suppressed in cosmic ray-dominated galaxy haloes

We study the impact of cosmic rays (CRs) on the structure of virial shocks, using a large suite of high-resolution cosmological FIRE-2 simulations accounting for CR injection by supernovae. In Milky Way-mass, low-redshift (z less than or similar to 1-2) haloes, which are expected to form 'hot haloes' with slowly cooling gas in quasi-hydrostatic equilibrium (with a stable virial shock), our simulations without CRs do exhibit clear virial shocks. The cooler phase condensing out from inflows becomes pressure confined to overdense clumps, embedded in low-density, volume-filling hot gas with volume-weighted cooling time longer than inflow time. The gas thus transitions sharply from cool free-falling inflow, to hot and thermal-pressure supported at approximately the virial radius (approximate to R-vir), and the shock is quasi-spherical. With CRs, we previously argued that haloes in this particular mass and redshift range build up CR-pressure-dominated gaseous haloes. Here, we show that when CR pressure dominates over thermal pressure, there is no significant virial shock. Instead, inflowing gas is gradually decelerated by the CR pressure gradient and the gas is relatively subsonic out to and even beyond R-vir. Rapid cooling also maintains subvirial temperatures in the inflowing gas within similar to R-vir.

Automated summary

This study examines the impact of cosmic rays on the structure of virial shocks using high-resolution cosmological simulations. It shows that in a specific mass and redshift range, when cosmic ray pressure dominates over thermal pressure, there is no significant virial shock, and inflowing gas is gradually decelerated by the cosmic ray pressure gradient.