Where is the Super-virial Gas? II: Insight from the Survey of Galactic Sightlines

TL;DR

This study investigates the location of super-virial temperature gas in the Milky Way, using X-ray spectra from Galactic X-ray binaries, and finds it is likely in the extended circumgalactic medium rather than the interstellar medium.

Contribution

The paper provides observational evidence that super-virial hot gas is not prevalent in the ISM but likely resides in the CGM, refining our understanding of galactic hot gas distribution.

Findings

Super-virial hot gas is not widespread in the ISM.

Detected hot gas lines are often time-variable and associated with XRBs.

Hot gas is likely located in the extended CGM or extraplanar regions.

Abstract

Recent observations have revealed a super-virial temperature gas phase at log(T/K) $\sim7$ in the Milky Way, challenging existing galaxy-formation models. This hot gas phase was discovered toward extragalactic absorption sightlines and blank-sky emission fields, both at high galactic latitudes. The location of this hot component is unknown; is it in the extended circumgalactic medium (CGM) or in the interstellar medium (ISM) instead? We analyzed X-ray spectra from Chandra's High-Energy Transmission Grating (HETG) observations of 27 Galactic X-ray binaries (XRBs) to investigate whether the hot gas component is present in the ISM. We searched for absorption lines of SXVI K$\alpha$, SiXIV K$\alpha$, and NeX K$\alpha$, which are the tell-tale signatures of the hot gas and which have been detected toward extragalactic sightlines. Of the 27 targets, these lines were detected in the spectra of only 7, with two sources displaying broad line features likely intrinsic to the XRB systems. Additionally, most of the detected lines are time-variable, reinforcing their likely association with the XRBs. Our results suggest that the super-virial temperature gas is not a widespread component of the ISM but may instead be located in extraplanar regions or the extended CGM, in line with some recent simulation results.