Global Hot Gas in and around the Galaxy

TL;DR

This paper reviews recent advances in understanding the hot interstellar medium around the Galaxy, highlighting how X-ray and UV observations reveal its properties, distribution, and role in galaxy evolution.

Contribution

It synthesizes observational data to characterize the global properties of the hot gas, emphasizing the spatial, thermal, and kinematic aspects of the medium.

Findings

Diffuse hot gas primarily resides in and around the Galactic disk and bulge.

The gas has a characteristic temperature of about 10^6 K.

Evidence suggests a larger, hotter circum-Galactic medium exists.

Abstract

The hot interstellar medium traces the stellar feedback and its role in regulating the eco-system of the Galaxy. I review recent progress in understanding the medium, based largely on X-ray absorption line spectroscopy, complemented by X-ray emission and far-UV OVI absorption measurements. These observations enable us for the first time to characterize the global spatial, thermal, chemical, and kinematic properties of the medium. The results are generally consistent with what have been inferred from X-ray imaging of nearby galaxies similar to the Galaxy. It is clear that diffuse soft X-ray emitting/absorbing gas with a characteristic temperature of $\sim 10^6$ K resides primarily in and around the Galactic disk and bulge. In the solar neighborhood, for example, this gas has a characteristic vertical scale height of $\sim 1$ kpc. This conclusion does not exclude the presence of a larger-scale, probably much hotter, and lower density circum-Galactic hot medium, which is required to explain observations of various high-velocity clouds. This hot medium may be a natural product of the stellar feedback in the context of the galaxy formation and evolution.