THE CO-TO-H2 CONVERSION FACTOR FROM INFRARED DUST EMISSION ACROSS THE LOCAL GROUP

By A. Leroy, A. Bolatto, K. Gordon, K. Sandstrom, P. Gratier, Erik Rosolowsky, C. Engelbracht, N. Mizuno, E. Corbelli, Y. Fukui and 1 more. 2011

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We estimate the conversion factor relating CO emission to H2 mass, αCO, in five Local Group galaxies that span approximately an order of magnitude in metallicity—M 31, M 33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the αCO that best allows a single gas-to-dust ratio (δGDR) to describe each system. This approach remains sensitive to CO-dark envelopes H2 surrounding molecular clouds. In M 31, M 33, and the LMC we find αCO ≈ 3–9 M☉ pc−2 (K km s−1)−1, consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log (O/H) ≈ 8.2 and 8.0), exhibit a much higher αCO. Our best estimates are αNGC6822CO ≈ 30 M☉ pc−2 (K km s−1)−1 and αSMCCO ≈ 70 M☉ pc−2 (K km s−1)−1. These results are consistent with the conversion factor becoming a strong function of metallicity around 12 + log (O/H) ∼ 8.4–8.2. We favor an interpretation where decreased dust shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, methodology): “This has been noted both in observations (e.g. Leroy et al. 2011; Genzel et al. 2012) and in other numerical models (Feldmann, Gnedin & Kravtsov 2011; Krumholz, Leroy & McKee 2011; Shetty et al. 2011b).”
Photodissociation and photoionisation of atoms and molecules of astrophysical interest (2017, methodology): “This parameter is also estimated from observations of local group Galaxies, and found to vary between about 50 and 500 (Draine et al. 2007; Leroy et al. 2011), and on small scales within the Milky Way, in one case falling to 88 and rising above 200 within one star-forming region (Liseau et al.…”
PHIBSS: Unified Scaling Relations of Gas Depletion Time and Molecular Gas Fractions (2017, methodology): “Then molecular gas masses are estimated assuming a gas-to-dust mass ratio (which can be a function of metallicity, see below, e.g. Leroy et al. 2011, Rémy–Ruyer et al. 2014, Eales et al. 2012, Magdis et al. 2011, 2012a, Magnelli et al. 2012a, Santini et al. 2014, Sargent et al. 2014, Béthermin et…”
ESTIMATING THE STAR FORMATION RATE AT 1 kpc SCALES IN NEARBY GALAXIES (2012, background): “…by the CO J = 1 → 0 flux measured by Young et al. (1995), Helfer et al. (2003), or Kuno et al. (2007). of metallicities (e.g., Moustakas et al. 2010) and observational evidence suggests that the CO-to-H2 conversion factor varies as a function of metallicity (e.g., see summary in Leroy et al. 2011).”
THE STATE OF THE GAS AND THE RELATION BETWEEN GAS AND STAR FORMATION AT LOW METALLICITY: THE SMALL MAGELLANIC CLOUD (2011, methodology): “Note that although the Galactic values are approximately appropriate on the small spatial scales in the CO-bright material (Bolatto et al. 2008), they are most likely inappropriate on the large scales (Rubio et al. 1993; Israel 1997; Israel et al. 2003; Leroy et al. 2007a, 2011).”
Dusty Star Forming Galaxies at High Redshift (2014, background): “Within the Milky Way and Local Group (aside from the Small Magallenic Cloud [SMC]), the conversion factor appears to display a remarkably narrow range of XCO ≈ 2–4 × 1020cm−2/K km s−1 (or αCO ≈ 3–6 M⊙ pc−2 (K km s−1)−1 Bloemen et al., 1986; Solomon et al., 1987; Blitz et al., 2007; Delahaye et al., ”
The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, methodology): “For the conversion factor from CO(2-1) to total molecular gas mass, we adopt a fixed ratio CO(2-1)/CO(1-0) = 0.7 (e.g. Gratier et al. 2010; Leroy et al. 2011; T. Saito et al. in prep.) and use the αCO factors provided by Sandstrom et al. (2013) when available (i.e. for NGC628, NGC3351, NGC3627,…”
THE MAGELLANIC MOPRA ASSESSMENT (MAGMA). I. THE MOLECULAR CLOUD POPULATION OF THE LARGE MAGELLANIC CLOUD (2011, methodology): “Leroy et al. (2011) use a dust-based method to deriveXCO = 3.0× 1020 cm−2 (K km s−1)−1 for the LMC, based on comparison ofSpitzerIR, NANTEN CO, and ATCA+Parkes HI maps.”
THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS): ENVIRONMENTAL DEPENDENCE OF GIANT MOLECULAR CLOUD PROPERTIES IN M51 (2014, background): “Bresolin et al. (2004), and Moustakas et al. (2010) both found a metallicity close to solar with only a shallow radial gradient of −(0.02 ± 0.01) dex kpc−1, so we do not expect metallicity-dependent variations in the conversion factor across the PAWS FoV (see, e.g., Leroy et al. 2011, 2012).”
The evolution of the dust temperatures of galaxies in the SFR–M∗ plane up to z ~ 2 (2013, methodology): “(6)–(8), assuming that Δlog (SSFR)MS and μ are independent, and assuming that the gas-to-dust relation of Leroy et al. (2011) holds at high redshift, we can thus predict from the gas phase the evolution of the dust temperature as a function of Δ log (SSFR)MS and μ: log (Tdust) = 0.5 × 1/(4 + β) ×…”
The ATLAS3D Project – XXVIII. Dynamically driven star formation suppression in early-type galaxies (2014, background): “XCO has been shown to vary as a function of metallicity (e.g. Wilson 1995; Bolatto et al. 2008; Leroy et al. 2011; Sandstrom et al. 2012)and in high-redshift starbursts (e.g. Solomon et al. 1997; Downes & Solomon 1998).”
The CO-to-H2 Conversion Factor (2013, methodology): “They find XCO ∝ Z−1.3 − Z−1.8, in approximate agreement with local measurements (Leroy et al. 2011; Schruba et al. 2012).”
PHIBSS: MOLECULAR GAS CONTENT AND SCALING RELATIONS IN z ∼ 1–3 MASSIVE, MAIN-SEQUENCE STAR-FORMING GALAXIES (2012, methodology): “In the Milky Way and nearby star forming galaxies with near solar metallicity, as well as in dense star forming clumps of lower mass, lower metallicity galaxies, the empirical CO 1-0 conversion factors determined with dynamical, dust and γ-ray calibrations are broadly consistent with a single value ”
THE EVOLVING INTERSTELLAR MEDIUM OF STAR-FORMING GALAXIES SINCE z = 2 AS PROBED BY THEIR INFRARED SPECTRAL ENERGY DISTRIBUTIONS (2012, methodology): “The method relies on measuring the total dust mass of a galaxy (Mdust) and assuming that it is proportional to Mgas (e.g., Leroy et al. 2011).”
MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES (2013, background): “There is good evidence that the CO-to-H2 conversion factor increases with decreasing metallicity and dust-togas ratio (see references in Israel 1997; Leroy et al. 2011; Bolatto et al. 2011).”
The CO-H2 Conversion Factor in Disc Galaxies and Mergers (2011, background): “…well (Blitz et al. 2007; Wolfire et al. 2010), though there may be some variations associated with metallicity (Wilson1995; Arimoto et al. 1996; Boselli et al. 2002; Rosolowsky et al. 2003; Israel 2005; Bell et al. 2006; Leroy et al. 2006; Bell et al. 2007; Bolatto et al. 2008; Leroy et al. 2011).”
THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES (2012, methodology): “At lower metallicities, a strong dependence of αCO on metallicity has been observed using the same technique we employ (Leroy et al. 2011).”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “…Low 2011; Krumholz et al. 2011b; Narayanan et al. 2011, 2012; 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…”