High Star Formation Efficiency in Barred Galaxy Centers Revealed with a New CO-to-H2 Conversion Factor Prescription

The CO-to-H2 conversion factor (alphaCO) is central to studying molecular gas and star formation in galaxies, and it is known to vary with environmental conditions. In particular, alphaCO can drop in galaxy centers by nearly an order of magnitude. The variation of alphaCO causes major uncertainties in current molecular gas measurements, and it interferes with accurately measuring star formation efficiency and molecular cloud properties. Using high-resolution and sensitive ALMA observations of multiple 12CO, 13CO, and C18O lines in several barred galaxy centers, we have found that the alphaCO decreases are primarily driven by CO opacity changes and thus shows strong correlations with observables like velocity dispersion and 12CO/13CO line ratio (Teng et al. 2022, 2023). Motivated by these results, we have constructed a new alphaCO prescription which accounts for emissivity effects in galaxy centers and verified it on a set of barred and non-barred galaxies with measured alphaCO values. Applying our new prescription to the PHANGS-ALMA Large Program dataset, we observe an overall 3x higher star formation efficiency in barred centers than in non-barred centers, which is an unprecedented trend that has been obscured by current alphaCO treatments. We discuss how the alphaCO variation and current prescriptions impact star formation efficiency and molecular gas concentration in various types of galaxies and galactic nuclei, leveraging the full PHANGS-ALMA sample of 65 nearby star-forming galaxies.