THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES

By K. Sandstrom, A. Leroy, F. Walter, A. Bolatto, K. Croxall, B. Draine, C. Wilson, M. Wolfire, D. Calzetti, Robert C. Kennicutt Jr. and 35 more. 2012

346 citations recorded by the source corpus.

We present ∼kiloparsec spatial resolution maps of the CO-to-H2 conversion factor (αCO) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for αCO and the DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H i column density to solve for both αCO and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, 12CO J = (2–1) maps from the IRAM 30 m large program HERACLES, and H i 21 cm line maps from THINGS. We use a fixed ratio between the (2–1) and (1–0) lines to present our αCO results on the more typically used 12CO J = (1–0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for αCO and the DGR. On average, αCO = 3.1 M☉ pc−2 (K km s−1)−1 for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of αCO as a function of galactocentric radius. However, most galaxies exhibit a lower αCO value in the central kiloparsec—a factor of ∼2 below the galaxy mean, on average. In some cases, the central αCO value can be factors of 5–10 below the standard Milky Way (MW) value of αCO, MW = 4.4 M☉ pc−2 (K km s−1)−1. While for αCO we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate αCO for studies of nearby galaxies.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/338b39a164d78757236841f6908f6b47c9201095

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In Astronomy and Astrophysics

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

In Atmospheric Science

1. S. Glover
2. A. Leroy
3. A. Bolatto
4. D. Narayanan
5. F. Galliano
6. Usm Munich
7. D. Kruijssen
8. R. Somerville

Citations in the corpus

• The Nine Lives of Cosmic Rays in Galaxies (2015, methodology): “The local-arms average is only 30 % larger than the mean found in external galaxies (Sandstrom 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, neutral): “Note that Sandstrom et al. (2013) find a similar trend for decreasing αCO with increasing metallicity when considering spatially distinct regions within nearby galaxies.”
• A z = 0 Multiwavelength Galaxy Synthesis. I. A WISE and GALEX Atlas of Local Galaxies (2019, background): “This is be expected from the lower dust-to-gas ratio and overall gas column densities in low mass galaxies (e.g., Sandstrom et al. 2013; Rémy-Ruyer et al. 2014).”
• The dynamical evolution of molecular clouds near the Galactic Centre - I. Orbital structure and evolutionary timeline (2014, background): “An additional issue is that gas and young stellar populations are generally probed using indirect tracers, the calibration of which has become a very active field of research (see e.g. Leroy et al. 2011; Sandstrom et al. 2013).”
• The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, methodology): “…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, NGC4254 and NGC4321).”
• PHIBSS: Unified Scaling Relations of Gas Depletion Time and Molecular Gas Fractions (2017, background): “…by dust in atomic gas (section 2.2 above and also sections 3.2 and 3.3 below), such that the dust and inferred gas masses in this redshift range overestimate the true molecular masses (e.g. Draine et al. 2007, Kennicutt et al. 2011, Dale et al. 2012, Eales et al. 2012, Sandstrom et al. 2013).”
• The dust content of galaxies from z = 0 to z = 9 (2016, methodology): “In our paper, we will also be quoting dust mass estimates derived using these more complex models, such as in Sandstrom et al. (2013), Rémy-Ruyer et al. (2014b), Ciesla et al. (2014) and Santini et al. (2014).”
• Star formation rates and efficiencies in the Galactic Centre (2017, background): “For example, several authors have shown that there is a gradient of decreasing gas-to-dust ratio with decreasing Galactocentric radius (Schlegel, Finkbeiner & Davis 1998; Watson 2011), a trend which has also been observed in several other star-forming galaxies (Sandstrom et al. 2013).”
• Gas-to-dust mass ratios in local galaxies over a 2 dex metallicity range (2013, methodology): “Moreover the small scatter (less than a factor of 2) observed by Sandstrom et al. (2013) can be due to the fact that they are probing very similar environments.”
• Dusty Star Forming Galaxies at High Redshift (2014, background): “This was seen first towards the Galactic Center (e.g. Oka et al., 1998; Strong et al., 2004), and has been noted in other nearby galactic nuclei as well (Sandstrom et al., 2013).”
• Cloud-scale Molecular Gas Properties in 15 Nearby Galaxies (2018, background): “A single conversion factor has the merit of showing our measurements directly, but we have good reason to believe that αCO varies across our sample (Blanc et al. 2013; Sandstrom et al. 2013).”
• The evolution of the dust and gas content in galaxies (2013, methodology): “As suggested by previous studies, focused either on local (e.g., Draine et al. 2007; Leroy et al. 2011; Smith et al. 2012; Corbelli et al. 2012; Sandstrom et al. 2013), z < 0.5 (e.g., James et al. 2002) or high-z galaxies (e.g., Zafar & Watson 2013; Chen et al. 2013, Cresci et al., in prep.)”
• Molecular line emission in NGC 1068 imaged with ALMA: I. An AGN-driven outflow in the dense molecular gas (2014, methodology): “Applying the linear dust/gas scaling ratio of Draine et al. (2007) (see also Sandstrom et al. 2013) to the gas-phase oxygen abundances measured in the central 2 kpc of NGC 1068 (∼ 12+log (O/H)∼ 8.8; Pilyugin et al. 2004, 2007), which yields a gas–to–dust mass ratio of ∼ 60+30−30, we estimate that…”