Deviations from hydrostatic equilibrium in the circumgalactic medium: spinning hot haloes and accelerating flows

TL;DR

This study uses simulations to show that the hot circumgalactic medium often deviates from hydrostatic equilibrium, exhibiting significant rotation and acceleration, especially in lower-mass haloes, challenging traditional assumptions.

Contribution

It provides new insights into the dynamics of the hot CGM, highlighting the importance of rotation and non-thermal motions, and compares simulation results with X-ray observations.

Findings

Hot CGM shows significant tangential motions and rotation.

Deviations from hydrostatic equilibrium increase in lower-mass haloes.

Feedback processes induce outward accelerations and buoyant motions.

Abstract

Hydrostatic equilibrium (HSE), where the thermal pressure gradient balances the force of gravity, is tested across a range of simulated EAGLE haloes from Milky Way L* haloes (M_200~10^12 Msol) to cluster scales. Clusters (M_200>=10^14 Msol) reproduce previous results with thermal pressure responsible for ~90% of the support against gravity, but this fraction drops for group-sized haloes (M_200~10^13 Msol) and is even lower (40-70%) for L* haloes between 0.1-0.3 R_200. Energy from feedback grows relative to the binding energy of a halo toward lower mass resulting in greater deviations from HSE. Tangential motions comprise the largest deviation from HSE in L* haloes indicating that the hot circumgalactic medium (CGM) has significant sub- centrifugal rotation and angular momentum spin parameters 2-3x higher than the dark matter spin parameters. Thermal feedback can buoyantly rise to the outer CGM of M_200<10^12 Msol haloes, both moving baryons beyond R_200 and feeding uncorrelated tangential motions. The resulting hot halo density and rotation profiles show promising agreement with X-ray observations of the inner Milky Way halo, and we discuss future observational prospects to detect spinning hot haloes around other galaxies. Acceleration and radial streaming motions also comprise significant deviations from HSE, especially net outward accelerations seen in L* and group haloes indicating active feedback. Black hole feedback acts in a preventative manner during the later growth of group haloes, applying significant accelerations via shocks that do not feed tangential motions. We argue that HSE is a poor assumption for the CGM, especially in the inner regions, and rotating baryonic hot haloes are a critical consideration for analytic models of the CGM.