Constraining density and metallicity of the Milky Way's hot gas halo from O VII spectra and ram-pressure stripping

Abstract

Milky Way's hot gaseous halo extends up to the Galactic virial radius ($\sim 200$ kpc) and contains a significant component of baryon mass of the Galaxy. The halo properties can be constrained from X-ray spectroscopic observations and from satellite galaxies' ram-pressure stripping studies. Results of the former method crucially depend on the gas metallicity assumptions while the latter one's are insensitive to them. Here, a joint analysis of both kinds of data is presented to constrain electron density and metallicity of the gas. The power law is assumed for the electron density radial profile, while for the metallicity, a common-used constant-metallicity assumption is relaxed by introducing of a physically motivated spherical profile. The model is fitted to a sample of 431 (18) sight lines for O VII emission (absorption) measurements and 7 electron density constraints from ram-pressure stripping studies. The best-fitting halo-associated electron density profile of $n_e\propto r^{-(0.9...1.1)}$ (where $r\gg1$ kpc is the galactocentric radius) is found. The metallicity is constrained as $Z \approx (0.1...0.7) Z_{\odot}$ (subscript $\odot$ represents the solar values) at $r>50$ kpc. These imply a total hot gas mass of $M \approx (2.4...8.7) \times 10^{10} M_{\odot}$, which accounts for $\sim(17...100)$ per cent of the Milky Way's missing baryon mass. The model uncertainties are discussed, and the results are examined in the context of previous studies.