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

We measure the CO-to-H-2 conversion factor (alpha(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 similar to 790 and similar to 610 independent measurements of alpha(CO) for CO (2-1) and (1-0), respectively. The mean values for alpha(CO (2-1)) and alpha(CO (1-0)) are 9.3-5.4+4.6 and 4.2(-2.0)(+1.9)M(circle dot) pc(-2)(Kkms(-1))(-1), respectively. The CO-intensity-weighted mean is 5.69 for alpha(CO (2-1)) and 3.33 for alpha(CO (1-0)). We examine how alpha(CO) scales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength ((U) over bar). Among them, (U) over bar, Sigma(SFR), and the integrated CO intensity (W-CO) have the strongest anticorrelation with spatially resolved alpha(CO). We provide linear regression results to alpha(CO) for all quantities tested. At galaxy-integrated scales, we observe significant correlations between alpha(CO) and W-CO, metallicity, (U) over bar, and Sigma(SFR). We also find that alpha(CO) in each galaxy decreases with the stellar mass surface density (Sigma(star)) in high-surface-density regions (Sigma(star) >= 100 M-circle dot pc(-2)), following the power-law relations alpha(CO(2-1))proportional to Sigma(-0.5)(star) and alpha(CO)(1-0)proportional to Sigma(-0.2)(star). The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease in alpha(CO) with increasing Sigma(star) 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 alpha(CO). The decrease in alpha(CO) at high Sigma(star) 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.