Hot Gaseous Halos of Nearby Disk Galaxies
  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 \ovi 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 $\sim 10$ 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 ($\gsim 10^2$ 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. 
Q. Daniel Wang