GASKAP Pilot Survey Science II: ASKAP Zoom Observations of Galactic 21-cm Absorption

TL;DR

This study uses ASKAP to observe 21-cm absorption spectra, revealing the structure and temperature of neutral atomic gas in different parts of the Galaxy, with implications for understanding the distribution of cold and warm gas phases.

Contribution

It provides new measurements of 21-cm absorption in the Galaxy's outer and inner regions, highlighting the scale height and temperature distribution of neutral atomic gas.

Findings

Cool atomic gas has a smaller scale height in the inner Galaxy.

The cool gas fraction remains roughly constant with Galactic radius.

The mean spin temperature is about 300 K, indicating one-third of HI mass is in the cool phase.

Abstract

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.