Spatial distribution of the Milky Way hot gaseous halo constrained by Suzaku X-ray observations

Abstract

The formation mechanism of the hot gaseous halo associated with the Milky Way Galaxy is still under debate. We report new observational constraints on the gaseous halo using 107 lines-of-sight of the Suzaku X-ray observations at $75^{\circ}<l<285^{\circ}$ and $|b|>15^{\circ}$ with a total exposure of 6.4 Ms. The gaseous halo spectra are represented by a single-temperature plasma model in collisional ionization equilibrium. The median temperature of the observed fields is 0.26 keV ($3.0\times10^6$ K) with a typical fluctuation of $\sim30$%. The emission measure varies by an order of magnitude and marginally correlates with the Galactic latitude. Despite the large scatter of the data, the emission measure distribution is roughly reproduced by a disk-like density distribution with a scale length of $\sim7$ kpc, a scale height of $\sim2$ kpc, and a total mass of $\sim5\times10^7$ $M_{\odot}$. In addition, we found that a spherical hot gas with the $\beta$-model profile hardly contributes to the observed X-rays but that its total mass might reach $\gtrsim10^9$ $M_{\odot}$. Combined with indirect evidence of an extended gaseous halo from other observations, the hot gaseous halo likely consists of a dense disk-like component and a rarefied spherical component; the X-ray emissions primarily come from the former but the mass is dominated by the latter. The disk-like component likely originates from stellar feedback in the Galactic disk due to the low scale height and the large scatter of the emission measures. The median [O/Fe] of $\sim0.25$ shows the contribution of the core-collapse supernovae and supports the stellar feedback origin.