Tracing dense and diffuse neutral hydrogen in the halo of the Milky Way
  We have combined observations of Galactic high-velocity HI from two surveys:
a very sensitive survey from the Green Bank 140ft Telescope with limited sky
coverage, and the less sensitive but complete Galactic All Sky Survey from the
64m Parkes Radio Telescope. The two surveys preferentially detect different
forms of neutral gas due to their sensitivity. We adopt a machine learning
approach to divide our data into two populations that separate across a range
in column density: 1) a narrow line-width population typical of the majority of
bright high velocity cloud components, and 2) a fainter, broad line-width
population that aligns well with that of the population found in the Green Bank
survey. We refer to these populations as dense and diffuse gas respectively,
and find that diffuse gas is typically located at the edges and in the tails of
high velocity clouds, surrounding dense components in the core. A fit to the
average spectrum of each type of gas in the Galactic All Sky Survey data
reveals the dense population to have a typical line width of ~20 km/s and
brightness temperature of ~0.3 K, while the diffuse population has a typical
line width of ~30 km/s and a brightness temperature of ~0.2 K. Our results
confirm that most surveys of high velocity gas in the Milky Way halo are
missing the majority of the ubiquitous diffuse gas, and that this gas is likely
to contribute at least as much mass as the dense gas.
