Hot Gas in the Galaxy

Abstract

Observations of the x-ray diffuse background suggest that a significant fraction of the volume of the Galaxy contains low-density million-degree gas. Large-scale angular structures seen in x-ray all-sky surveys imply the existence of at least three nearb y bubbles of hot gas, Sco-Cen, Orion-Eridanus, and Mono-Gem, in addition to x-ray emitting gas located in the local cavity. The near-isotropy of the high-latitude x-ray background in the 0.5 — 1 keVrange away from identified bubbles is consistent with a large-scale-height few-million-degree x-ray emitting halo or emission from the intragroup medium of the Local Group. Absorption features seen in the x-ray background angular distribution in the 0.1 – 0.28 keV range also place some irregular regions of l ower temperature gas in the halo. In this brief review, I will summarize our knowledge of these hot regions.

I will also touch on our knowledge of the spectrum of the local cavity emission. High-spectral resolution observations reveal that the lowenergy spectrum (E < 0.3 keV) from the local cavity is not consistent with models of the x-ray emission from collisi onal-equilibrium ionization plasmas or from non-equilibrium ionization plasmas. Improved models, particularly those incorporating charge transfer onto solar wind ions, would be helpful in this low energy range. Additional data, particularly an all-sky sur vey at high spectral resolution over the 0.1 – 1 keV range, would also be helpful.