Resolved Measurements of the CO-to-H${}_2$ Conversion Factor in 37 Nearby Galaxies

Abstract

We measure the CO-to-H${}_2$ conversion factor (${\alpha }_{\mathrm{CO}}$) in 37 galaxies at 2~kpc resolution, using dust surface density inferred from far-infrared emission as a tracer of the gas surface density and assuming a constant dust-to-metals ratio. In total, we have $\sim 790$ and $\sim 610$ independent measurements of ${\alpha }_{\mathrm{CO}}$ for CO (2-1) and (1-0), respectively. The mean values for ${\alpha }_{\mathrm{CO}(2-1)}$ and ${\alpha }_{\mathrm{CO}(1-0)}$ are $9.3_{-5.4}^{+4.6}$ and $4.2_{-2.0}^{+1.9}{M}_{\odot }p{c}^{-2}(Kkm{s}^{-1}{)}^{-1}$, respectively. The CO-intensity-weighted mean for ${\alpha }_{\mathrm{CO}(2-1)}$ is 5.69, and 3.33 for ${\alpha }_{\mathrm{CO}(1-0)}$. We examine how ${\alpha }_{\mathrm{CO}}$ scales with several physical quantities, e.g.\ star-formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength ($\overline{U}$). Among them, $\overline{U}$, ${\Sigma }_{\mathrm{SFR}}$, and integrated CO intensity ($W_{\mathrm{CO}}$) have the strongest anti-correlation with spatially resolved ${\alpha }_{\mathrm{CO}}$. We provide linear regression results to \aco for all quantities tested. At galaxy integrated scales, we observe significant correlations between ${\alpha }_{\mathrm{CO}}$ and $W_{\mathrm{CO}}$, metallicity, $\overline{U}$, and ${\Sigma }_{\mathrm{SFR}}$. We also find that the normalized ${\alpha }_{\mathrm{CO}}$ decreases with stellar mass surface density (${\Sigma }_{\star }$) in the high surface density regions (${\Sigma }_{\star }\ge 100{\mathrm{M}}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}$), following the power-law relations ${\alpha }_{\mathrm{CO}(2-1)}\propto {\Sigma }_{\star }^{-0.5}$ and ${\alpha }_{\mathrm{CO}(1-0)}\propto {\Sigma }_{\star }^{-0.2}$. The power-law index is insensitive to the assumed dust-to-metals ratio. (abridged)

27 pages, 13 figures, 4 tables, submitted to ApJ