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

We measure the CO-to-H2 conversion factor (αCO) 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 αCO for CO (2–1) and (1–0), respectively. The mean values for αCO (2–1) and αCO (1–0) are 9.3−5.4+4.6 and 4.2−2.0+1.9M⊙pc−2(Kkms−1)−1, respectively. The CO-intensity-weighted mean is 5.69 for αCO (2–1) and 3.33 for αCO (1–0). We examine how αCO scales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength (U¯). Among them, U¯, ΣSFR, and the integrated CO intensity (WCO) have the strongest anticorrelation with spatially resolved αCO. We provide linear regression results to αCO for all quantities tested. At galaxy-integrated scales, we observe significant correlations between αCO and WCO, metallicity, U¯, and ΣSFR. We also find that αCO 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 αCO(2–1)∝Σ⋆−0.5 and αCO(1–0)∝Σ⋆−0.2. The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease in αCO 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 αCO. The decrease in αCO 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.