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 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 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 H i mass.

Automated summary

Using the Australian Square Kilometre Array Pathfinder, this study investigates the cool phase of neutral atomic gas in both the inner and outer regions of the Milky Way galaxy. It is found that the cool atomic gas in the inner region has a smaller scale height, similar to the molecular gas and super-thin stellar population. In the outer region, the cool atomic gas is mixed with warm neutral medium, with the cool fraction remaining roughly constant with Galactic radius. Analysis of the emission brightness temperature and absorption ratio suggests that the fraction of gas mass in the cool phase is approximately one-third of the total H i mass.