Where is the Super-Virial Gas? The Supply from hot inflows

TL;DR

This study uses galaxy simulations to identify and analyze super-virial gas near Milky Way-like galaxies, revealing that infall and compressive heating are key drivers, with stellar feedback and cosmic rays playing minor roles.

Contribution

It demonstrates that super-virial gas is primarily heated by infall and compression, not stellar feedback or cosmic rays, providing new insights into galactic hot gas dynamics.

Findings

Super-virial gas mass is 1-2×10^7 solar masses within 20 kpc.

Infalling gas reaches super-virial temperatures before cooling and joining the disk.

Stellar feedback and cosmic rays have minimal impact on super-virial gas heating.

Abstract

In an effort to understand the presence of super-virial gas detected in the Milky Way, we present our findings from isolated galaxy simulations of Milky Way-like systems using GIZMO with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. It unveils the presence of a significant super-virial temperature ( $T>6\times10^6$K) gas component within 20 kpc from the galactic center. This super-virial gas has a mass of $1-2\times10^7$ ${\rm M_\odot}$ and is found close to the disk, where typical gas densities are $0.004-0.01 \, \rm cm^{-3}$. We find that some of the virial gas ($T\sim10^6$K) forms a rotating hot inflow, where gravitational energy is converted to heat mainly via compressive heating. This process causes gas infalling close to the rotation axis to reach super-virial temperatures just before cooling and joining the disk. Stellar feedback heating accounts for less than 1% of the super-virial gas, indicating its minimal influence despite expectations. Even in scenarios with no stellar feedback effects considered, abundant super-virial gas persists, highlighting the dominance of alternative heating mechanisms. We also show that cosmic rays do not have a significant effect on heating the gas to a super-virial temperature. Our study illuminates the intricate dynamics of hot virial and super-virial gas surrounding Milky Way-like galaxies, emphasizing the prominent role of infall-driven and compressive heating processes in shaping thermal evolution.