Some observational consequences of residual H I in galactic molecular clouds

The presence of a residue of cold H I in galactic molecular clouds has several demonstrable effects on low-latitude H I line profiles. Molecular clouds are responsible for narrow, well-defined H I self-absorption features, for much of the small-scale scatter in measured H I terminal velocities, and for some of the general lumpiness in the H I emission structure. In the inner Galaxy, they appear to have a general blanketing effect on H I intensities observed near the terminal velocity, thus changing fundamentally the entire positive-velocity structure of the H I line profiles and their macroscopic optical-depth properties. On the smallest accessible observational scales, the perceived emission characteristics of CO and H I are often strongly anticorrelated; as the observed scales become larger, the emission characteristics of the two species become increasingly similar.