“Missing Link” Clouds in the Southern Galactic Plane Survey

We present an automated routine to search for HI self-absorption featureswithin the Southern Galactic Plane Survey (SGPS). The data were taken with theAustralia Telescope Compact Array (ATCA) and the Parkes Radio Telescope andencompass 3 deg by 105 deg of sky in the Galactic plane. We apply our routineto this entire region and derive spin temperatures and column densities for 70of the larger HISA complexes, finding spin temperatures ranging from 6-41 Kwith HI number densities of a few cm^-3. These `missing link' clouds fill inthe spin temperature and density gaps between dense molecular clouds anddiffuse atomic clouds. We compare the HI emission with 12CO emission and findthat 60% of detected HI self-absorption is correlated in space and in velocitywith a molecular counterpart. This is potentially due to a molecular/atomic gastransition. We also compare HI self-absorption with Galactic spiral arms anddiscuss the possibility of using it as a spiral arm tracer.Comment: 37 pages, 13 figures, 2 tables; Accepted for publication in Ap