The Properties of the Galactic Hot Gaseous Halo from X-Ray Emission

Abstract

The extended hot X-ray emitting gaseous halo of the Milky Way has an optical depth ∼1 for the dominant emission lines of O vii and O viii , which are used to infer the halo properties. To improve on halo gas properties, we treat optical depth effects with a Monte Carlo radiative transfer model, which leads to slightly steeper density profiles ( β ≈ 0.5 ) than if optical depths effects were ignored. For the preferred model where the halo is rotating on cylinders at 180 km s −1 , independent fits to both lines lead to identical results, where the core radius is 2.5 kpc and the turbulent component of the Doppler- b parameter is 100–120 km s −1 ; the turbulent pressure is 20% of the thermal pressure. The fit is improved when emission from a disk is included, with a radial scale length of 3 kpc (assumed) and a fitted vertical scale height of approximately 1.3 kpc. The disk component is a minor mass constituent and has low optical depth, except at low latitudes. The gaseous mass is 3 – 4 × 10 10 M ⊙ within 250 kpc , similar to our previous determinations and significantly lower than the mass of the missing baryons, which is 1.7 × 10 11 M ⊙ .