ALMA-FACTS: Variations in CO(2-1)/CO(1-0) Across Twelve Nearby Spiral Galaxies

We will present early results from a high-resolution analysis of the CO(2-1)/CO(1-0) line ratio, R21, across the disks of twelve nearby spiral galaxies. We use new ALMA CO(1-0) observations from the Fundamental CO(1-0) Transition Survey (FACTS), and re-image CO(2-1) data from PHANGS. Higher ratios suggest higher densities and temperatures. The sample includes barred and unbarred, and flocculent galaxies. We find that R21 varies systematically as a function of structure. The barred spiral galaxies follow a general trend: R21 is high in the center, low along the bar, increases at the bar ends, and then declines towards the outskirts of the disk. R21 in unbarred galaxies fluctuate along the spiral arms. The barred galaxies also have spiral arms beyond the bar, which show similar fluctuations. The gas around HII regions typically have higher ratios. These results show that R21 varies systematically as a function of galactic structure and star formation activity, which should be taken into account when we interpret the relation between gas and star formation.