Virial shocks are suppressed in cosmic ray-dominated galaxy haloes

Abstract

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 ≲ 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 (≈Rvir), 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 Rvir. Rapid cooling also maintains subvirial temperatures in the inflowing gas within ∼Rvir.

The impact of cosmic rays on dynamical balance and disk-halo interaction in Lstar disk galaxies

“…However, in CR+, the thermal pressure at a large height is weakened significantly (Salem et al 2016;Butsky & Quinn 2018;Hopkins et al 2021c)). This is because CR feedback modifies the multiphase gas flows and the phase structure of the CGM (Hopkins et al 2021a;Ji et al 2020Ji et al , 2021. We discuss this in the next section and later show that the near-disk hot gas content is still proportional to the SFR in §5.2.1, consistent with a star formation driven origin.…”

“…The more prominent difference between Hydro+ and CR+ is the pressure support at a large height. In Hydro+, thermal pressure takes over from kinetic pressure at a height z 10 kpc as expected from hot halos in L galaxies without CRs (Ji et al 2020). However, in CR+, the thermal pressure at a large height is weakened significantly (Salem et al 2016;Butsky & Quinn 2018;Hopkins et al 2021c)).…”

“…Hot gas from the bubbles is not able to escape from the halo (Muratov et al 2015), possibly due to strong gravity and hot gas confinement (see discussion in §4.2.2 and Hopkins et al 2021a). Additional infall of hot gas is supplied from the hot CGM (produced by virial shocks; see Ji et al 2021).…”

“…This CR energy density is comparable to the thermal energy density of halo gas at 10 kpc in our non-CR simulations with n ∼ 10 −3 cm −3 and T ∼ 10 6 K, so thermal (or kinetic) pressure is not required to support the gas away from galactic disks. In fact, CR pressure can be important even out to virial radii if the diffusion coefficient is constant (see Ji et al 2020;Hopkins et al 2021a), effectively supporting a large fraction of the halo gas.…”

“…In this work, we select the set of CR runs from Hopkins et al (2020b) calibrated to reproduce the observed γ-ray emission to gas density relation (Chan et al 2019;Hopkins et al 2020b), which also matches the MW grammage constraint (Hopkins et al 2021c). They yield a better match to the cool CGM properties (Ji et al 2020) when compared to simulations without CRs.…”