A Global View of Molecule-Forming Clouds in the Galaxy

Abstract

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.

arXiv comment2 pages, 1 figure, to appear in proceedings of IAU Symposium 315, "From Interstellar Clouds to Star-Forming Galaxies: Universal Processes?"

References in corpus (8)

• The Milky Way in Molecular Clouds: A New Complete CO Survey
• The Neutral Atomic Phases of the ISM in the Galaxy
• The Dark Molecular Gas
• The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• The VLA Galactic Plane Survey
• The Southern Galactic Plane Survey: HI Observations and Analysis
• A Self-Absorption Census of Cold HI Clouds in the Canadian Galactic Plane Survey
• An Automated Method for the Detection and Extraction of HI Self-Absorption in High-Resolution 21cm Line Surveys

Cited by in corpus (1)

• Populating H${}_2$ and CO in galaxy simulation with dust evolution