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

Abstract

ABSTRACT Milky Way’s (MW) hot gaseous halo extends up to the Galactic virial radius (∼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) sightlines for O vii emission (absorption) measurements and seven electron density constraints from ram-pressure stripping studies. The best-fitting halo-associated electron density profile of ne ∝ r−(0.9...1.1) (where r ≫ 1 kpc is the Galactocentric radius) is found. The metallicity is constrained as Z ≃ (0.1...0.7) Z⊙ (subscript ⊙ represents the solar values) at $r \gtrsim 50$ kpc. These imply a total hot gas mass of M ≃ (2.4...8.7) × 1010 M⊙, which accounts for ∼(17...100) per cent of the MW’s missing baryon mass. The model uncertainties are discussed, and the results are examined in the context of previous studies.