Cold Atomic Hydrogen in the Galaxy

The cold atomic phase of the interstellar medium is readily discerned as H I self-absorption (HISA) shadows against warmer background H I emission. HISA studies have benefited significantly from recent large-scale, high-resolution surveys of Galactic H I. These surveys reveal a rich, intricate population of HISA features, many dominated by structure at arcminute scales. Although quite cold, a lot of HISA lacks CO emission, contrary to the expectations of theory. Weak HISA is visible almost everywhere that conditions allow it to occur, but stronger HISA is concentrated into cloudlike features. Many of these occur at velocities consistent with their being made visible by spiral density waves, and it's possible the cold gas may trace evolving material that is forming molecules after passing through a spiral shock. This work is supported by a grant from the Natural Sciences and Engineering Research Council of Canada.