The Hidden Realm of Cold Interstellar Hydrogen

A large fraction of the neutral atomic hydrogen (H I) in the Galaxy is hidden from direct view. Arcminute resolution 21 cm line maps from the Canadian Galactic Plane Survey (CGPS) reveal numerous cold hydrogen clouds as H I self-absorption (HISA) shadows against warmer background emission. HISA has traditionally been thought to trace the small fraction of atomic gas in molecular clouds. However the HISA features in our survey do not correlate in strength with CO emission, and they display only a limited physical association with CO. It may be that the CO is a less reliable tracer of molecular hydrogen than is usually assumed, or perhaps we are seeing different mixtures of H I and H₂ in different clouds. If the latter is true, HISA may be tracking gas in transition from the atomic to the molecular state; many of the strongest HISA in our survey is found at velocities expected of gas downstream of the Perseus spiral shock, where molecular condensation is expected to occur. In our ongoing investigation of this and other possible models of cold interstellar hydrogen, we consider the properties of the HISA clouds and their distribution in space revealed by the CGPS data.