Zoom info: https://stanford.zoom.us/j/98604058568
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 (αCO), 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 αCO, particularly in galaxy centers where αCO 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 12CO, 13CO, and C18O lines in several barred galaxy centers, we found that αCO 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 αCO prescription which accounts for emissivity effects in galaxy centers and verified it on a set of barred and non-barred galaxies with measured αCO values from dust. Applying our new prescription to 65 galaxies from the PHANGS survey, we found an overall 3x higher star formation efficiency in barred galaxy centers than in non-barred centers, and such a trend is obscured when using a MW αCO or other existing prescriptions. Our results suggest that the high star formation rates observed in barred centers are not simply due to an increased amount of molecular gas but also an enhanced star formation efficiency compared to non-barred centers or disk regions.