The CO-to-H2 Conversion Factor

Abstract

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 H2 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-H2 “conversion factor,” XCO, in different environments. In the Milky Way disk, we recommend a conversion factor of 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 both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that XCO increases with decreasing metallicity, turning up sharply below metallicity ∼1/3–1/2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line model...