Abstract We present early results from a high-resolution analysis (∼100–200 pc) of the CO(2–1)/CO(1–0) line ratio in 12 nearby galaxies. We use new Atacama Large Millimeter/submillimeter Array (ALMA) CO(1–0) observations from the Fundamental CO(1–0) Transition Survey (FACTS), and re-imaged CO(2–1) data from PHANGS. We make empirical classifications based on the optical and molecular gas morphologies, which show clear systematic trends in the variation of R 21 as a function of galactic structure. The sample includes barred, unbarred, and flocculent galaxies. The barred spiral galaxies follow a general trend when the gas exists significantly: R 21 is high in the center, low along the bar, increases at the bar ends, and then lowers beyond the bar end or flattens in the outer parts of the disk. The structure dependence suggests the importance of galactic dynamics on molecular gas evolution, and consequently on star formation, in galaxies. R 21 fluctuates in the spiral arms for both barred and unbarred galaxies. Areas around HII regions in some cases appear to show more high-ratio gas. Together, R 21 varies systematically as a function of galactic structure, dynamics, and star formation activity.
Lee et al. (2026) studied this question.