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

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[140.03] ``Missing Link" Clouds in the Southern Galactic Plane Survey (SGPS)

D. W. Kavars, J. M. Dickey (University of Minnesota), N. M. McClure-Griffiths (Australia Telescope National Facility), B. M. Gaensler (Harvard-Smithsonian Center for Astrophysics), A. J. Green (University of Sydney)

We present an analysis of HI self-absorption features (HISA) within the Southern Galactic Plane Survey (SGPS). The data was taken with the Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope and encompasses 2 degrees by 105 degrees of sky in the Galactic Plane. We apply an automated search routine to the entire SGPS region, deriving distances, densities and spin temperatures for all HISA features found. In our analysis we select 71 of the larger HISA complexes, finding spin temperatures of 7-41 K and densities of a few hundred cm-3. With these values, HISA represents a population of cold `missing link' clouds that fill the artificial gap between the colder, denser molecular clouds and the warmer, less dense diffuse atomic clouds. A comparison to molecular gas finds that 50-60% of HISA features have a 12CO counterpart, although the intensity of molecular gas can vary dramatically from one cloud to the next. We propose that these clouds are in a molecular/atomic transition and the fraction of molecular gas depends on the transition stage of the cloud. HISA provides an observational tool with which to study the least molecular of the molecular clouds. We also study the link between HISA clouds and Galactic spiral arms. Due to interactions with spiral density waves it can be expected that 12CO and HISA should be well correlated within the spiral arms, but an anti-correlation should exist in the inter-arm region. Our analysis finds no conclusive evidence that HISA with a molecular counterpart is constrained to the spiral arms. This work was supported by NSF grant AST-0307603 to the University of Minnesota.

The author(s) of this abstract have provided an email address for comments about the abstract: dkavars@astro.umn.edu

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© 2004. The American Astronomical Society.