The CO-to-H2 Conversion Factor

2013/01/31 by Alberto D. Bolatto, Mark Wolfire, M. G. Wolfire +1 · 2,200 citations
Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Conversion factor #Galaxies: Formation, Evolution, Phenomena #Galaxy #Interstellar medium #Metallicity #Milky Way #Physics #Redshift #Star formation #Stellar, planetary, and galactic studies #astro-ph.GA

paper · pdf · doi:10.1146/annurev-astro-082812-140944

published in Annual Review of Astronomy and Astrophysics 51(1), 207-268 (Annual Reviews) · To appear in ARAA, Annual Reviews of Astronomy and Astrophysics, ARAA issue 51. This instance is compiled in ApJ format, 35 pages, 12 figures (most in color)

arxiv created 2013/06/08 · openalex publication_date 2013/06/29 · arxiv updated 2015/06/12 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06

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 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 both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that X CO 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 modeling and dust observations, X CO appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower X CO is also present in the overwhelmingly molecular ISM of starburst galaxies, where several lines of evidence point to a lower CO-to-H 2 conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments.

Citations

• Molecular Gas at High Redshift
• Star Formation in the Milky Way and Nearby Galaxies
• Theory of Star Formation
• The Chemical Composition of the Sun

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