Missing link'' clouds in the Southern Galactic Plane Survey

We present an automated routine to search for H I self- absorption features within the Southern Galactic Plane Survey ( SGPS). The data were taken with the Australia Telescope Compact Array ( ATCA) and the Parkes Radio Telescope and encompass 3 degrees x 105 degrees of sky in the Galactic plane. We apply our routine to this entire region and derive spin temperatures and column densities for 70 of the larger HISA complexes, finding spin temperatures ranging from 6 to 41 K with H I number densities of a few cm(-3). These missing link clouds fill in the spin temperature and density gaps between dense molecular clouds and diffuse atomic clouds. We compare the H I emission with (CO)-C-12 emission and find that similar to 60% of detected H I self- absorption is correlated in space and in velocity with a molecular counterpart. This is potentially due to a molecular/ atomic gas transition. We also compare H I self- absorption with Galactic spiral arms and discuss the possibility of using it as a spiral arm tracer.