Virial shocks are suppressed in cosmic ray-dominated galaxy halos
  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 massive ($M_{\rm halo} \gtrsim
10^{11}\,M_{\odot}$), low-redshift ($z\lesssim 1-2$) halos, which are expected
to form "hot halos" 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 over-dense clumps, embedded in low-density, volume-filling
hot gas whose cooling time is much 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 ($\approx R_{\rm vir}$), and the
shock is quasi-spherical. With CRs, we previously argued that halos in this
particular mass and redshift range build up CR-pressure-dominated gaseous
halos. 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 $R_\mathrm{vir}$. Rapid cooling also maintains sub-virial
temperatures in the inflowing gas within $\sim R_\mathrm{vir}$.
