Density and metallicity of the Milky-Way circumgalactic gas

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.Comment: 5 pages, 6 figures, mn2e.cls . V2: discussion extended, results unchanged. Version accepted by MNRAS Letter