GASKAP Pilot Survey Science II: ASKAP Zoom Observations of Galactic
  21-cm Absorption
  Using the Australian Square Kilometre Array Pathfinder to measure 21-cm
absorption spectra toward continuum background sources, we study the cool phase
of the neutral atomic gas in the far outer disk, and in the inner Galaxy near
the end of the Galactic bar at longitude 340 degrees. In the inner Galaxy the
cool atomic gas has a smaller scale height than in the solar neighborhood,
similar to the molecular gas and the superthin stellar population in the bar.
In the outer Galaxy the cool atomic gas is mixed with the warm, neutral medium,
with the cool fraction staying roughly constant with Galactic radius. The mean
spin temperature, i.e. the ratio of the emission brightness temperature to the
absorption, is roughly constant for velocities corresponding to Galactic radius
greater than about twice the solar circle radius. The ratio has a value of
about 300 K, but this does not correspond to a physical temperature in the gas.
If the gas causing the absorption has kinetic temperature of about 100 K, as in
the solar neighborhood, then the value 300 K indicates that the fraction of the
gas mass in this phase is one-third of the total HI mass.
