UC San Diego

Star formation in galaxies is governed by the amount of molecular gas and the efficiency that gas is converted into stars. However, assessing the amount of molecular gas relies on the CO-to-H2 conversion factor (alpha_CO), which is known to vary with molecular gas conditions like density, temperature, and dynamical state -- the same conditions that also alter star formation efficiency. The variation of alpha_CO, particularly in galaxy centers where alpha_CO can drop by nearly an order of magnitude, thus causes major uncertainties in current molecular gas and star formation efficiency measurements. Using ALMA observations of multiple low-J 12CO, 13CO, and C18O lines in several barred galaxy centers, we found that alpha_CO is primarily driven by CO opacity changes and therefore shows strong correlations with observables like velocity dispersion and 12CO/13CO line ratio. Motivated by these results, we have constructed a new alpha_CO prescription which accounts for emissivity effects in galaxy centers and verified it on a set of barred and non-barred galaxies with measured alpha_CO values from dust. Applying our new prescription to 65 galaxies from the PHANGS-ALMA survey, we found an overall three times higher star formation efficiency in barred galaxy centers than in non-barred galaxy centers, and such a trend is obscured when using a constant alpha_CO or other existing prescriptions. Our results suggest that the high star formation rate observed in barred galaxy centers is due to an enhanced star formation efficiency compared to non-barred galaxy centers or the disk regions, rather than a substantially increased amount of molecular gas in barred galaxy centers.