Limits on Hot Galactic Halo Gas from X-ray Absorption Lines

TL;DR

This study uses X-ray absorption lines to investigate the presence and extent of hot gaseous halos around the Milky Way, finding that observed absorption is mainly from the Galactic disk with negligible halo contribution.

Contribution

First observational constraints on the Milky Way's hot gaseous halo using high-resolution X-ray spectroscopy, indicating minimal halo gas contribution to metal line absorption.

Findings

Absorption lines are consistent with the interstellar medium in the Galactic disk.

Halo contribution to OVII absorption is constrained to be very low.

Hot gas in the halo, if present, has a negligible impact on metal line absorption.

Abstract

Although the existence of large-scale hot gaseous halos around massive disk galaxies have been theorized for a long time, there is yet very little observational evidence. We report the Chandra and XMM-Newton grating spectral detection of OVII and NeIX Kalpha absorption lines along the sight-line of 4U 1957+11. The line absorption is consistent with the interstellar medium in origin. Attributing these line absorptions to the hot gas associated with the Galactic disk, we search for the gaseous halo around the Milky Way by comparing this sight-line with more distant ones (toward X-ray binary LMC X-3 and the AGN Mrk 421). We find that all the line absorptions along the LMC X-3 and Mrk 421 sight-lines are attributable to the hot gas in a thick Galactic disk, as traced by the absorption lines in the spectra of 4U~1957+11 after a Galactic latitude dependent correction. We constrain the OVII column density through the halo to be N(OVII) < 5E15 cm^{-2} (95% confidence limit), and conclude that the hot gas contribution to the metal line absorptions, if existing, is negligible.