A Global View of Molecule-Forming Clouds in the Galaxy
  We have mapped cold atomic gas in 21cm line HI self-absorption (HISA) at
arcminute resolution over more than 90% of the Milky Way's disk. To probe the
formation of H2 clouds, we have compared our HISA distribution with CO J=1-0
line emission. Few HISA features in the outer Galaxy have CO at the same
position and velocity, while most inner-Galaxy HISA has overlapping CO. But
many apparent inner-Galaxy HISA-CO associations can be explained as chance
superpositions, so most inner-Galaxy HISA may also be CO-free. Since standard
equilibrium cloud models cannot explain the very cold HI in many HISA features
without molecules being present, these clouds may instead have significant
CO-dark H2.
