Milky Way Disk-Halo Transition in HI: Properties of the Cloud Population
  Using 21cm HI observations from the Parkes Radio Telescope's Galactic All-Sky
Survey, we measure 255 HI clouds in the lower Galactic halo that are located
near the tangent points at 16.9 < l < 35.3 degrees and |b| < 20 degrees. The
clouds have a median mass of 700 Msun and a median distance from the Galactic
plane of 660 pc. This first Galactic quadrant (QI) region is symmetric to a
region of the fourth quadrant (QIV) studied previously using the same data set
and measurement criteria. The properties of the individual clouds in the two
quadrants are quite similar suggesting that they belong to the same population,
and both populations have a line of sight cloud-cloud velocity dispersion of
sigma_cc ~ 16 km/s. However, there are three times as many disk-halo clouds at
the QI tangent points and their scale height, at h=800 pc, is twice as large as
in QIV. Thus the observed line of sight random cloud motions are not connected
to the cloud scale height or its variation around the Galaxy. The surface
density of clouds is nearly constant over the QI tangent point region but is
peaked near R~4 kpc in QIV. We ascribe all of these differences to the
coincidental location of the QI region at the tip of the Milky Way's bar, where
it merges with a major spiral arm. The QIV tangent point region, in contrast,
covers only a segment of a minor spiral arm. The disk-halo HI cloud population
is thus likely tied to and driven by large-scale star formation processes,
possibly through the mechanism of supershells and feedback.
