The Disk-Halo Connection and Where Has All The Gas Gone?
  The wealth of data in the past decades, and especially in the past 15 years
has transformed our picture of the gas around the Milky Way and other spiral
galaxies. There is good evidence for extraplanar gas that is a few kpc in
height and is seen in all gaseous phases: neutral; warm atomic; and hot, X-ray
emitting gas. This medium is seen not only around the Milky Way, but other
spiral galaxies and it is related to the star formation rate, so it is likely
produced by the activity in the disk through a galactic fountain. More extended
examples of halo gas are seen, such as the HVC around the Milky Way and around
M31. This gas is typically 10-20 kpc from the galaxy and is not seen beyond 50
kpc. This gas is most likely being accreted. A hot dilute halo (1E6 K) is
present with a similar size, although its size is poorly determined. An ongoing
controversy surrounds the relative amounts of outflow from the disk and
accretion onto galaxies such as the Milky Way. There is good evidence for
accretion of cold material onto the Milky Way and other galaxies, but it is not
clear if there is enough to modify the gas content and star formation
properties in the disk. The reservoir of accretion material is as yet
unidentified. Some of these findings may be related to the issue that galaxies
are baryon-poor: their baryon to dark matter ratio is well below the
cosmological value. The absence of baryons may be due to extremely violent
outflow events in the early stages of galaxy formation. We may be able to
understand this stage of galaxy evolution by applying our deepening
understanding of our local disk-halo environment.
