AAS 205th Meeting, 9-13 January 2005
Session 140 Cold Gas and Molecular Clouds
Poster, Thursday, January 13, 2005, 9:20am-4:00pm, Exhibit Hall

## [140.01] The Physical Properties of a Cold HI Feature Observed Transitioning from Absorption to Emission

C. R. Kerton (Iowa State University)

Large scale surveys of neutral hydrogen HI 21-cm line emission from the Galactic plane at approximately 1' resolution have revealed an extensive population of HI self-absorption features, i.e., presumably cold HI seen in absorption against warmer background HI. Determining the spin temperature of such features primarily depends upon some sort of radiative transfer modeling that constrains various parameters. We present here observations from the high-latitude extension of the Canadian Galactic Plane Survey (CGPS) of a HISA feature that is seen to transition from absorption into emission thus allowing a straightforward determination of both its spin temperature and optical depth. Using a simple radiative transfer model we derive a spin temperature of Ts=45±~K and an optical depth of \tau=~0.7. We also show how observations made at the transition point can be used to tightly constrain the results from multicomponent radiative transfer models. One of the main scientific projects in Galactic astronomy with the Arecibo L-band Feed Array (GALFA) is the detection and characterization of cold HI structures seen both in self-absorption and in emission. Since GALFA observations will include both high and low Galactic latitude regions we expect that many additional cold HI features will be seen transitioning from absorption to emission as the background HI emission drops off at higher latitude and thus be amenable to the analysis techniques presented here.

This research is funded by Iowa State University. Funding for the CGPS is provided by NSERC Canada and NRC Canada.

Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.