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.