Resolved Measurements of the CO-to-H₂ Conversion Factor in 37 Nearby Galaxies

Abstract

We measure the CO-to-H₂ conversion factor (α _CO) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from far-infrared emission as a tracer of the gas surface density and assuming a constant dust-to-metal ratio. In total, we have ∼790 and ∼610 independent measurements of α _CO for CO (2–1) and (1–0), respectively. The mean values for α _{CO (2–1)} and α _{CO (1–0)} are ${9.3}_{-5.4}^{+4.6}$ and ${4.2}_{-2.0}^{+1.9}\,{M}_{\odot }\,{\mathrm{pc}}^{-2}\,{({\rm{K}}\,\mathrm{km}\,{{\rm{s}}}^{-1})}^{-1}$ , respectively. The CO-intensity-weighted mean is 5.69 for α _{CO (2–1)} and 3.33 for α _{CO (1–0)}. We examine how α _CO scales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength ( $\overline{U}$ ). Among them, $\overline{U}$ , Σ_SFR, and the integrated CO intensity (W _CO) have the strongest anticorrelation with spatially resolved α _CO. We provide linear regression results to α _CO for all quantities tested. At galaxy-integrated scales, we observe significant correlations between α _CO and W _CO, metallicity, $\overline{U}$ , and Σ_SFR. We also find that α _CO in each galaxy decreases with the stellar mass surface density (Σ_⋆) in high-surface-density regions (Σ_⋆ ≥ 100 M _⊙ pc^‑2), following the power-law relations ${\alpha }_{\mathrm{CO}\,(2\mbox{--}1)}\propto {{\rm{\Sigma }}}_{\star }^{-0.5}$ and ${\alpha }_{\mathrm{CO}\,(1\mbox{--}0)}\propto {{\rm{\Sigma }}}_{\star }^{-0.2}$ . The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease in α _CO with increasing Σ_⋆ as a result of higher velocity dispersion compared to isolated, self-gravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction in α _CO. The decrease in α _CO at high Σ_⋆ is important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge "Milky Way–like" to "starburst-like" conversion factors.