Hot gas
Extended X-ray emission from the central 300×200 pc of the Galaxy reveals that the CMZ is embedded in a corona of hot, low-density gas that probably occupies most of the volume of the central zone, and is roughly coextensive with the CMZ (Kawai et al 1988; Koyama et al 1989, 1996; Yamauchi et al 1990; Sunyaev et al 1993; Markevitch et al 1993). The hot gas is evidenced by both the X-ray continuum and the 6.7 keV line emission from helium-like iron (24 times ionized). A recent mosaic of the continuum emission made with the Chandra X-Ray Observatory. It illustrates the mix of diffuse emission and large numbers of point sources, most of which are X-ray binary stars. Until recently, it was thought that the gas temperature in the X-ray emitting gas was ∼10⁸ K because of the strength of the 6.7 keV line, but that interpretation has recently been called into question by Wang et al (2002). With the Chandra X-Ray Observatory, they have mapped a 2^o by 0.8^o region about the Galactic Center with 1'' spatial resolution, and have found that much of the 6.7 keV line emission emanates from discrete stellar sources (presumably close binary systems), rather than from the diffuse structures. Wang et al argue that the temperature of the hot gas is therefore more like 10⁷ K, a much less demanding constraint in terms of the requisite heating source. One of the most important consequences of lowering the temperature to 10⁷ K is that the gas is bound to the Galaxy. At 10⁸ K, the gas would be unbound, implying a thermal, galactic wind. In this case, the galactic wind could well be a major sink for the gas migrating inwards through the CMZ. The XMM satellite observatory should soon provide images of the full extent of the coronal gas bubble.