Journal or Book Title
EAS Publications Series
I review studies of the hot gaseous medium in and around nearby normal disk galaxies, including the Milky Way. This medium represents a reservoir of materials required for lasting star formation, a depository of galactic feedback (e.g., stellar mass loss and supernovae), and an interface between the interstellar and intergalactic media. Important progress has been made recently with the detection of X-ray absorption lines in the spectra of X-ray binaries and AGNs. The X-ray absorption line spectroscopy, together with existing X-ray emission and far-UV O IV absorption measurements now allows for the first time to characterize the global spatial, thermal, and chemical properties of hot gas in the Galaxy. The results are generally consistent with those inferred from X-ray imaging of nearby edge-on galaxies similar to the Milky Way. Observed diffuse X-ray emitting/absorbing gas does not extend significantly more than 10 kpc away from galactic disks/bulges, except in nuclear starburst or very massive galaxies. The X-ray cooling rate of this gas is generally far less than the expected supernova mechanical energy input alone. So the bulk of the energy is "missing". On the other hand, evidence for a large-scale (102 kpc) hot gaseous halo around the Milky Way to explain various high-velocity clouds is mounting. The theoretical argument for ongoing accretion of intergalactic gas onto disk galaxies is also compelling. I discuss possible solutions that reconcile these facts. In particular, large-scale hot gaseous halos appear to be low in metallicity, hence X-ray emission. The metal enrichment in the intergalactic medium may be substantially non-uniform; fast-cooling clumps of relatively high metallicity may have largely dropped out and may partly account for high-velocity clouds. In addition, ongoing galactic mechanical energy feedback is likely important in balancing the cooling of the halos and may be strong enough to produce galactic winds in bulge-dominated galaxies.
Wang, QD, "Hot Gaseous Halos of Nearby Disk Galaxies" (2007). EAS Publications Series. 1018.