X-Ray Absorption from the Milky Way Halo and the Local Group

TL;DR

This study uses X-ray absorption measurements toward bright AGNs to determine whether the Milky Way's halo or a diffuse Local Group medium accounts for observed hot gas, supporting a Galactic Halo origin.

Contribution

It provides observational evidence favoring a Galactic Halo model over a Local Group medium for million-degree gas at zero redshift.

Findings

Supports Galactic Halo model over Local Group medium

Estimates halo radius between 15-110 kpc

Finds gas density and mass consistent with a 20 kpc halo

Abstract

Million degree gas is present at near-zero redshift and is due either to a gaseous Galactic Halo or a more diffuse but very massive Local Group medium. We can discriminate between these models because the column densities should depend on location in the sky, either relative to the Galaxy bulge or to the M31-Milky Way axis. To search for these signatures, we measured the OVII Kalpha absorption line strength toward 25 bright AGNs, plus LMC X-3, using XMM-Newton RGS archival data. The data are in conflict with a purely Local Group model, but support the Galactic Halo model. The strongest correlation is between the OVII equivalent widths and the ROSAT background emission measurement in the R45 band (0.4-1 keV), for which OVII emission makes the largest single contribution. This suggests that much of the OVII emission and absorption are cospatial, from which the radius of a uniform halo appears to lie the range 15-110 kpc. The present data do not constrain the type of halo gas model and an equally good fit is obtained in a model where the gas density decreases as a power-law, such as r^(-3/2). For a uniform halo with a radius of 20 kpc, the electron density would be 9E-4 cm^(-3), and the gas mass is 4E8 Msolar. The redshift of the four highest S/N OVII measurements is consistent with a Milky Way origin rather than a Local Group origin.