Dark Hydrogen and the Cold Interstellar Medium

The cold atomic phase of interstellar gas is visible as 21cm-line H I self-absorption (HISA) shadows against warmer background H I emission. Although HISA has been used for decades to study cold clouds, recent large-scale, high-resolution surveys have revolutionized our ability to image these objects. A rich, intricate population of HISA features is now revealed, with abundant structure at arcminute scales. Weak HISA is visible almost everywhere that conditions allow it to occur, while stronger HISA is concentrated into cloudlike features. Much of the HISA lacks CO emission, which poses a problem for equilibrium ISM models that require molecular gas at such cold temperatures. However, HISA may trace evolving gas instead: many strong features occur at the velocities expected for atomic gas starting to form molecules downstream of spiral arm shocks. This work is supported by a grant from the Natural Sciences and Engineering Research Council of Canada.