GASKAP pilot survey science. II. 21cm abs.

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 super-thin 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 the Galactic radius. The ratio of the emission brightness temperature to the absorption, i.e., 1-e^-{tau}^, is roughly constant for velocities corresponding to Galactic radius greater than about twice the solar circle radius. The ratio has a value of about 300K, but this does not correspond to a physical temperature in the gas.