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+1.9M⊙pc−2(Kkms−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 ( U¯ ). Among them, U¯ , ΣSFR, 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, U¯ , and ΣSFR. 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 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 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.