GASKAP Pilot Survey Science II: ASKAP Zoom Observations of Galactic 21-cm Absorption
J.M. Dickey, J.M. Dempsey, N.M. Pingel, N.M. McClure-Griffiths, K., Jameson, J.R. Dawson, H. D\'enes, S.E. Clark, G. Joncas, D. Leahy, Min-Young, Lee, M.-A. Miville-Desch\^enes, S. Stanimirovi\'c, C.D. Tremblay, J. Th. van, Loon

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…
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Taxonomy
TopicsAstronomy and Astrophysical Research · Astrophysics and Star Formation Studies · Galaxies: Formation, Evolution, Phenomena
