A Sub-solar Fe/O, logT~7.5 Gas Component Permeating the Milky Way's CGM

TL;DR

This study characterizes the hot, highly ionized gas in the Milky Way's circumgalactic medium using X-ray spectra, revealing a homogeneous hot phase with super-solar metallicity and implications for Galactic feedback.

Contribution

First detection and detailed modeling of a super-solar metallicity hot gas component in the Milky Way's CGM using stacked X-ray spectra.

Findings

Hot gas phase has logT~7.5 and is inhomogeneous in column density.

Hot phase requires over-abundances of elements relative to solar.

No Fe L-shell lines detected, indicating high O/Fe ratio.

Abstract

Our study focuses on characterizing the highly ionized gas within the Milky Way's (MW) Circumgalactic Medium (CGM) that gives rise to ionic transitions in the X-ray band 2 - 25 \AA. Utilizing stacked \Chandra/\ACISS\ \MEG\ and \LETG\ spectra toward QSO sightlines, we employ the self-consistent hybrid ionization code PHASE to model our data. The stacked spectra are optimally described by three distinct gas phase components: a \warm\ (\logT\ $\sim$ 5.5), \warmhot\ (\logT\ $\sim 6$), and \hot\ (\logT\ $\sim$ 7.5) components. These findings confirm the presence of the \hot\ component in the MW's CGM indicating its coexistence with a \warm\ and a \warmhot\ gas phases. We find this \hot\ component to be homogeneous in temperature but inhomogeneous in column density. The gas in the \hot\ component requires over-abundances relative to solar to be consistent with the Dispersion Measure (DM) from the Galactic halo reported in the literature. {For the hot phase we estimated a DM = $55.1^{+29.9}_{-23.7}$ pc cm$^{-3}$}. We conclude that this phase is either enriched in Oxygen, Silicon, and Sulfur, or has metallicity {over 6} times solar value, or a combination of both. We do not detect Fe L-shell absorption lines, implying O/Fe $\geq$ 4. The non-solar abundance ratios found in the super-virial gas component in the Galactic halo suggest that this phase arises from Galactic feedback.