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

TL;DR

This study models the Milky Way's hot gaseous halo using X-ray emission lines, accounting for optical depth effects, and finds a rotating halo with specific density and turbulence properties, contributing to understanding galaxy baryon content.

Contribution

It introduces a Monte-Carlo radiative transfer model to better infer halo properties from X-ray emission lines, considering optical depth effects and including a disk component.

Findings

Halo gas density profile is steeper with optical depth effects.

Halo mass within 250 kpc is 3-4×10^{10} solar masses.

Turbulent pressure is 20% of thermal pressure.

Abstract

The extended hot X-ray emitting gaseous halo of the Milky Way has an optical depth $\sim1$ for the dominant emission lines of \ion{O}{7} and \ion{O}{8}, 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 ($\beta \approx 0.5$) 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 $3-4\times10^{10}\,M_{\odot}$ within $250\,\mathrm{kpc}$, similar to our previous determinations and significantly less than the missing baryons of $1.7\times10^{11}\,M_{\odot}$.