Density and metallicity of the Milky-Way circumgalactic gas

TL;DR

This study combines X-ray spectroscopy and satellite galaxy observations to constrain the metallicity and density profile of the Milky Way's circumgalactic gas, revealing a metallicity decline and a relatively flat density profile.

Contribution

It integrates two observational methods to jointly determine the metallicity and density distribution of the circumgalactic medium, addressing previous uncertainties.

Findings

Metallicity decreases to about 0.1 Z_Sun in outer halo regions.

Gas density profile is relatively flat, falling as r^{-0.45 to -0.75}.

Reconciliation of different observational data sets achieved.

Abstract

The halo of the Milky-Way circumgalactic gas extends up to the virial radius of the Galaxy, ~250 kpc. The halo properties may be deduced from X-ray spectroscopic observations and from studies of the ram-pressure stripping of satellite dwarf galaxies. The former method is more precise but its results depend crucially on the assumed metallicity of the circumgalactic gas; the latter one does not need these assumptions. Here, the information from both approaches is combined to constrain observationally the gas metallicity and density as functions of the galactocentric distance. It is demonstrated that the two kinds of data could be reconciled if the metallicity decreased to Z~0.1Z_Sun in the outer parts of the extended halo. The corresponding gas density profile is rather flat, falling as r^{-(0.45...0.75)} at large galactocentric distances r.