Neutral hydrogen self -absorption in the Milky Way Galaxy

To develop a better understanding of the cold neutral medium phase of the interstellar medium, we present a detailed analysis of neutral hydrogen self-absorption (HISA) clouds in the Milky Way Galaxy. These HISA clouds are in the Southern Galactic Plane Survey (SGPS), spanning the region l = 253°--358° and |b| ≤ 1.3°, and in the VLA Galactic Plane Survey (VGPS), spanning the region l = 18°--67° and |b| ≤ 1.3°--2.3°. The SGPS and VGPS have an angular resolution of ∼1 arcminute and a velocity channel spacing of 0.82 km s^-1.

With the recent completion of these surveys, we can study HISA features across the Galaxy at a much better resolution and sensitivity than any previous work. To analyze HISA in detail, catalogs of clouds of all sizes, including those undetectable by eye alone, are required. We present an automated search routine to detect all HISA clouds in the SGPS. We compare HISA to CO data and find some HISA clouds associated with CO, but others have no associated CO. This suggests that HISA clouds are in a transition between molecular and atomic gas, bridging the gap between dense molecular clouds and warmer, diffuse atomic clouds. HISA thus plays an important role in the overall evolution of the Galaxy. To study this transition further, we present observations of the OH molecule toward a select sample of HISA clouds in the VGPS, using the Green Bank Telescope (GBT). We present an analysis of the molecular properties of this sample, including a derivation of an OH to H₂ conversion factor and H₂ to H I abundance ratios. We discuss the complex relationship between H I, OH, ¹²CO, and ¹³CO emission.

Finally we present a statistical analysis comparing HISA with infrared data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) project. The GLIMPSE data reveal a large number of compact, dark infrared clouds believed to be in the early stages of star formation. If GLIMPSE clouds are associated with HISA, they provide valuable information on the evolution of HISA clouds.