A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law

By D. Narayanan, Mark R. Krumholz, E. Ostriker, L. Hernquist. 2011

295 citations recorded by the source corpus.

The most common means of converting an observed CO line intensity into a molecular gas mass requires the use of a conversion factor (XCO). While in the Milky Way this quantity does not appear to vary significantly, there is good reason to believe that XCO will depend on the larger-scale galactic environment. With sensitive instruments pushing detections to increasingly high redshift, characterizing XCO as a function of physical conditions is crucial to our understanding of galaxy evolution. Utilizing numerical models, we investigate how varying metallicities, gas temperatures and velocity dispersions in galaxies impacts the way CO line emission traces the underlying H2 gas mass, and under what circumstances XCO may differ from the Galactic mean value. We find that, due to the combined effects of increased gas temperature and velocity dispersion, XCO is depressed below the Galactic mean in high surface density environments such as ultraluminous infrared galaxies (ULIRGs). In contrast, in low-metallicity environments, XCO tends to be higher than in the Milky Way, due to photodissociation of CO in metal-poor clouds. At higher redshifts, gas-rich discs may have gravitationally unstable clumps that are warm (due to increased star formation) and have elevated velocity dispersions. These discs tend to have XCO values ranging between present-epoch gas-rich mergers and quiescent discs at low z. This model shows that on average mergers do have lower XCO values than disc galaxies, though there is significant overlap. XCO varies smoothly with the local conditions within a galaxy, and is not a function of global galaxy morphology. We combine our results to provide a general fitting formula for XCO as a function of CO line intensity and metallicity. We show that replacing the traditional approach of using one constant XCO for starbursts and another for discs with our best-fitting function produces star formation laws that are continuous rather than bimodal, and that have significantly reduced scatter.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/2e76cee41661b5a795d1cc6414dd7010ec41effb

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In Astronomy and Astrophysics

1. P. Hopkins
2. L. Hernquist
3. R. Somerville
4. R. Dav'e
5. Mark R. Krumholz
6. D. Narayanan
7. E. Quataert
8. A. Dekel

Citations in the corpus

• COMBINED CO AND DUST SCALING RELATIONS OF DEPLETION TIME AND MOLECULAR GAS FRACTIONS WITH COSMIC TIME, SPECIFIC STAR-FORMATION RATE, AND STELLAR MASS (2014, background): “From conversion ‘factor’ to conversion ‘function’: From theoretical considerations the CO conversion factor in equation (4) is expected to be a function of several physical parameters (Narayanan et al. 2011, 2012, Feldmann et al. 2012 a,b).”
• simba: Cosmological simulations with black hole growth and feedback (2019, background): “During Cosmic Noon, it has long been seen that cosmological models tend to underpredict the main sequence amplitude (Daddi et al. 2007; Davé 2008; Narayanan et al. 2012; Sparre et al. 2015; Somerville & Davé 2015), typically by a factor of 2− 3.”
• Star formation in galaxy mergers with realistic models of stellar feedback and the interstellar medium (2012, background): “Narayanan et al. (2012) find that when applying a functional form for the CO–H2 conversion factor which depends on the physical conditions in the ISM to the observed galaxies in Fig.”
• Star formation in semi-analytic galaxy formation models with multiphase gas (2015, unknown): “6 steepens (Narayanan et al. 2012).”
• CO excitation of normal star forming galaxies out to $z=1.5$ as regulated by the properties of their interstellar medium (2014, methodology): “Also, an equivalent scaling withΣSFR is found by Narayanan et al (2012) using simulations.”
• Dusty Star Forming Galaxies at High Redshift (2014, methodology): “There is still a debate in the literature as to whether the CO–H2 conversion factor is bimodal or continuous (e.g. Daddi et al., 2010b; Genzel et al., 2010; Narayanan et al., 2012b).”
• The CO-to-H2 Conversion Factor (2013, methodology): “Global models ofXCO using hydrodynamic simulations to investigate the variation due to galactic environment are carried out by Feldmann, Gnedin & Kravtsov (2012) and Narayanan et al. (2011, 2012).”
• Star Formation in the Milky Way and Nearby Galaxies (2012, methodology): “Indeed, Narayanan et al. (2012) use a grid of model galaxies to infer a smooth function of I(CO): αCO = min[6.3, 10.7 × I(CO) −0.32 ]Z ′−0.65, where Z ′ is the metallicity divided by the solar value.”
• THE EVOLVING INTERSTELLAR MEDIUM OF STAR-FORMING GALAXIES SINCE z = 2 AS PROBED BY THEIR INFRARED SPECTRAL ENERGY DISTRIBUTIONS (2012, background): “…parameters than metallicity, such as the compactness and/or the clumpiness of the ISM of normal galaxies, and if the ISM conditions of MS galaxies do vary significantly, then αCO could smoothly decrease as we depart from the MS, and move towards the starburst regime (e.g., Narayanan et al. 2012).”
• Stellar and Quasar Feedback in Concert: Effects on AGN Accretion, Obscuration, and Outflows (2015, unknown): “The observations in Figure 2 are best fits from Narayanan et al. (2012) based on a variable XCO factor.”
• MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES (2013, methodology): ““Σ = 100” reflects a typical surface density that is often assumed and observed for extragalactic GMCs (e.g., Bolatto et al. 2008; Narayanan et al. 2012).”
• Molecular Clouds in the Milky Way (2015, background): “The underluminous nature of clouds in the outer Galaxy was identified by Digel et al. (1990), who attributed it to a reduced CO emissivity per H2 molecule (larger XCO) in outer Galaxy clouds in response to reduced gas temperatures and metallicities (Narayanan et al. 2012).”
• Molecular hydrogen abundances of galaxies in the EAGLE simulations (2015, background): “…conversation factor were on average lower in galaxies with large abundances of H2, as suggested by the theoretical studies of Lagos et al. (2012), Narayanan et al. (2012) and Popping et al. (2014), the H2 masses would be overestimated by Walter et al. (2014), improving the consistency with the…”
• Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations (2014, background): “…field, is still far from being settled both observationally and theoretically (Boselli et al. 200, Bell et al. 2007, Bolatto et al. 2008, Liszt et al. 2010, Leroy et al. 2011, Shetty et al. 2011a,b, Narayanan et al. 2012, Kennicutt & Evans 2012, Bolatto et al. 2013, Sandstrom et al. 2013).”