A Neutral Hydrogen Self‐Absorption Cloud in the Southern Galactic Plane Survey

Abstract

Using data from the Southern Galactic Plane Survey (SGPS) we analyze an H I self-absorption cloud centered on l = 3180, b = -05 with velocity v = -1.1 km s-1. The cloud was observed with the Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope and is at a near-kinematic distance of 400 pc, with derived dimensions of 5 × 11 pc. We apply two different methods to find the optical depth and spin temperature. In both methods we find upper limit spin temperatures ranging from 20 to 25 K and lower limit optical depths of ~1. We look into the nature of the H I emission and find that 60%-70% originates behind the cloud. We analyze a second cloud at the same velocity, centered on l = 319°, b = 04, with an upper limit spin temperature of 20 K and a lower limit optical depth of 1.6. The similarities in spin temperature, optical depth, velocity, and spatial location are evidence that the clouds are associated, possibly as one large cloud consisting of smaller clumps of gas. We compare H I emission data with 12CO emission and find a physical association of the H I self-absorption cloud with molecular gas.