A Global View of Molecule-Forming Clouds in the Galaxy

We have mapped cold atomic gas in 21cm line H i 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 H i in many HISA features without molecules being present, these clouds may instead have significant CO-dark H₂.