4:00–5:00 pm ERC 517
Host: Hsiao-Wen Chen
Eltha Yu-Hsuan Teng (University of California San Diego) "Revealing the Drivers of CO-to-H2 Conversion Factor Variation and its Impact on Star Formation Efficiency across Nearby Galaxies"
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 (α_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 α_CO, particularly in galaxy centers where α_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 12CO, 13CO, and C18O lines in several barred galaxy centers, we found that α_CO is primarily driven by CO opacity changes and therefore shows strong correlations with observables like velocity dispersion and 12CO/13CO line ratio [1, 2]. Motivated by these results, we have constructed a new α_CO prescription which accounts for emissivity effects in galaxy centers and verified it on a set of barred and non-barred galaxies with measured α_CO values from dust . Applying our new prescription to 65 galaxies from the PHANGS-ALMA 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 α_CO or other existing prescriptions. Our results suggest that the high star formation rate observed in barred centers is 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.
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