The CO-to-H2 Conversion Factor

By A. Bolatto, M. Wolfire, A. Leroy. 2013

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CO line emission represents the most accessible and widely used tracer of the molecular interstellar medium. This renders the translation of observed CO intensity into total H2 gas mass critical to understand star formation and the interstellar medium 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 XCO = 2×10 20 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 modeling and dust observations, XCO 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 XCO is also present in the overwhelmingly molecular interstellar medium of starburst galaxies, where several lines of evidence point to a lower CO-to-H2 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. Subject headings: ISM: general — ISM: molecules — galaxies: ISM — radio lines: ISM

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/2ba3fbeca26e177f52b495e28130ec7320427923

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Citations in the corpus

• AGN wind scaling relations and the co-evolution of black holes and galaxies (2017, background): “This is usually done by assuming a proper conversion factor αCO, which can be a function of density, metallicity, gas distribution etc. (see Bolatto et al. 2013, for a review).”
• CO-dark gas and molecular filaments in Milky Way-type galaxies (2014, background): “Galactic value of XCO,gal = 2 × 1020 cm−2K−1km−1s in each pixel (Bolatto et al. 2013).”
• Physical Models of Galaxy Formation in a Cosmological Framework (2014, background): “CO measurements are currently the best tracer of molecular gas content, although there remain significant uncertainties in the conversion factor from CO to H2 (XCO; Bolatto et al. 2013), particularly to higher redshifts.”
• Cloud-scale Molecular Gas Properties in 15 Nearby Galaxies (2018, methodology): “Despite a general understanding of the likely variations in αCO (Bolatto et al. 2013), we still lack a quantitative, observationally verified prescription for αCO that applies at cloud scales.”
• Evolution of Interstellar Medium, Star Formation, and Accretion at High Redshift (2017, methodology): “We believe this is more correct than the value obtained from Galactic gamma ray surveys (α = 2 × 1020) (see Bolatto et al. 2013) since the latter requires the questionable assumptions: 1) that the cosmic rays which produce the ∼ 2 MeV gamma rays by interaction with the gas fully penetrate the GMCs…”
• Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations (2014, background): “In recent years it has become evident that theXCO conversion factor is not universal, but instead changes with the physical properties of the ISM (e.g. Kennicutt & Evans 2012 and the recent review of Bolatto et al. 2013).”
• The fast molecular outflow in the Seyfert galaxy IC 5063 as seen by ALMA (2015, background): “…thick and dense medium and commonly used for ULIRG, and 0.2× 1020 cm−2 (K km s−1)−1 (corresponding to αCO = 0.34, the most conservative case) derived for optically thin cases and suggested for turbulent gas associated with winds (see the discussion in Bolatto et al. 2013 and in Geach et al. 2014).”
• COMBINED CO AND DUST SCALING RELATIONS OF DEPLETION TIME AND MOLECULAR GAS FRACTIONS WITH COSMIC TIME, SPECIFIC STAR-FORMATION RATE, AND STELLAR MASS (2014, methodology): “2521 Santini, P., Maiolino, R., Magnelli, B., et al. 2014, A&A, 562, 30 Sargent, M.T., Béthermin, M., Daddi, E., & Elbaz, D. 2012, ApJ, 747, 31 Sargent, M.Y., Béthermin, M., Daddi, E., et al. 2014, ApJ, 793, 19 Schiminovich, D., Wyder, T. K., Martin, D. C., et al. 2007, ApJS, 173, 315 Schruba, A., L”
• AGN outflows and feedback twenty years on (2018, neutral): “8–4, depending on if representative values for local spiral galaxies or mergers are assumed[63].”
• ISM MASSES AND THE STAR FORMATION LAW AT Z = 1 TO 6: ALMA OBSERVATIONS OF DUST CONTINUUM IN 145 GALAXIES IN THE COSMOS SURVEY FIELD (2015, methodology): “…restricted the samples to objects with high stellar mass (Mstellar = 2 × 1010 − 4 × 1011M ); thus we are not probing lower metallicity systems where the dust-to-gas abundance ratio is likely to drop or where there could be significant molecular gas without CO (see Bolatto et al. 2013).”
• REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, positive): “For this reason, we assign αCO values to those 122 galaxies lacking an observational constraint with the following prescription from Bolatto et al. (2013), which is based on modeling in Wolfire et al. (2010): αCO(Z ′) αCO(Z′=1) = exp ( +4.0∆AV Z′ AV,MW ) exp ( −4.0∆AV AV,MW ) .”
• Dusty Star Forming Galaxies at High Redshift (2014, background): “We note that an excellent in depth review of the topic has recently been written by Bolatto et al. (2013).”
• STARS, GAS, AND DARK MATTER IN THE SOLAR NEIGHBORHOOD (2015, methodology): “Their density estimate was based on a CO to H2 conversion factor of 2.7 1020´ cm 2- /(K km s 1)- ; using the value of 2 0.6 1020 ´ cm 2- /(K km s 1- ) suggested by Bolatto et al. (2013) reduces the central density to 0.15 cm 3- .”
• The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “…Shetty et al. 2011a,b; Feldmann et al. 2012a,b) and observations (Bolatto et al. 2011; Leroy et al. 2011; Genzel et al. 2012; Leroy et al. 2013; Bolatto et al. 2013) suggest that, at low metallicity dark, CO-free gas can completely dominate the gas budget of molecular clouds, leaving the…”
• ANDROMEDA'S DUST (2013, positive): “The center of M31 has ΣH ≈ 1.8 × 106 M kpc−2 averaged over 18 Using a conversion factor XCO(J = 1 → 0) = 2 × 1020 H2 cm−2 ( K km s−1)−1 (Bolatto et al. 2013).”