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The molecular phase of the interstellar medium (ISM) hosts star formation. Measuring its properties, e.g. density and kinematics, is key to understanding star formation and ISM properties. The low-J CO emission is the most frequently used tracer for molecular ISM, and the CO-to-H2 conversion factor interprets the observed CO emission to molecular gas mass. With the accurate measurements of CO emission by ALMA, our understanding of the CO-to-H2 conversion factor has become the limiting factor in studying star-forming ISM. Chiang et al. (2024) assembled multi-wavelength data from 37 nearby galaxies and measured the conversion factor at 2 kpc resolution. They found that the conversion factor in each galaxy anti-correlates with the stellar mass surface density (top panels). This trend persists when we average over all galaxies, especially at surface densities above 100 solar mass per parsec square (middle panel). When normalized in each galaxy, the conversion factor scales with the stellar mass surface density as a power law with power-law indices of -0.5 and -0.2 for CO (2-1) and CO (1-0), respectively. This finding provides the missing piece of an empirical tracer for the change in CO excitation condition. Together with studies in "CO-dark" gas and CO line ratios, we provide a complete prescription for translating low-J CO emissions in Schinnerer & Leroy 2024, ARA&A, 62, 369.

I-Da Chiang et al. (2024), ApJ, 964, 18