The CO-to-H 2 Conversion Factor

CO line emission represents the most accessible and widely used tracer of the molecular ISM. This renders the translation of observed CO intensity into total H 2 gas mass critical to understanding star formation and the ISM in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H 2 “conversion factor,” X CO , in different environments. In the Milky Way disk, we recommend a conversion factor of [Formula: see text] cm −2 (K km s −1 ) −1 with ±30% uncertainty. Studies of other “normal galaxies” return similar values in Milky Way–like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are…