Gaseous Galaxy Halos
  Galactic halo gas traces inflowing star formation fuel and feedback from a
galaxy's disk and is therefore crucial to our understanding of galaxy
evolution. In this review, we summarize the multi-wavelength observational
properties and origin models of Galactic and low redshift spiral galaxy halo
gas. Galactic halos contain multiphase gas flows that are dominated in mass by
the ionized component and extend to large radii. The densest, coldest halo gas
observed in neutral hydrogen (HI) is generally closest to the disk (< 20 kpc),
and absorption line results indicate warm and warm-hot diffuse halo gas is
present throughout a galaxy's halo. The hot halo gas detected is not a
significant fraction of a galaxy's baryons. The disk-halo interface is where
the multiphase flows are integrated into the star forming disk, and there is
evidence for both feedback and fueling at this interface from the temperature
and kinematic gradient of the gas and HI structures. The origin and fate of
halo gas is considered in the context of cosmological and idealized local
simulations. Accretion along cosmic filaments occurs in both a hot (> 10^5.5 K)
and cold mode in simulations, with the compressed material close to the disk
the coldest and densest, in agreement with observations. There is evidence in
halo gas observations for radiative and mechanical feedback mechanisms,
including escaping photons from the disk, supernova-driven winds, and a
galactic fountain. Satellite accretion also leaves behind abundant halo gas.
This satellite gas interacts with the existing halo medium, and much of this
gas will become part of the diffuse halo before it can reach the disk. The
accretion rate from cold and warm halo gas is generally below a galaxy disk's
star formation rate, but gas at the disk-halo interface and stellar feedback
may be important additional fuel sources.
