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

Abstract We measure the CO-to-H 2 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 ⊙ pc − 2 ( K km 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 ( U ¯ ). Among them, 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, 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 α CO ( 2 – 1 ) ∝ Σ ⋆ − 0.5 and α CO ( 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.