ALMA FACTS. II. Large Scale Variations in the ¹²CO(J = 2 – 1) to ¹²CO(J = 1 – 0) Line Ratio in Nearby Galaxies

Abstract We present 12 CO( J = 1 – 0) mapping observations over ∼1/2 of the optical disk of 12 nearby galaxies from the Fundamental CO 1–0 Transition Survey of nearby galaxies (FACTS), using the ALMA Total Power array. Variations in the 12 CO( J = 2 − 1)/ 12 CO( J = 1 − 0) line ratio r 21 are investigated. The luminosity-weighted r 21 of the 11 sample galaxies ranges from 0.52 to 0.69 with an average of 0.61. We use position–velocity diagrams along the major axis and tilted ring models to separate the normal rotating galactic disk from kinematic outliers that deviate from pure circular rotation. We find that r 21 is systematically higher in outliers compared to the disk. We compare r 21 between SA, SAB, and SB galaxies, and find no significant difference in the average r 21 depending on the presence of galactic bars. We find, however, that the radial gradient in r 21 is bimodal, where a group containing all SA galaxies prefer constant or very shallow r 21 gradients out to 40% of the optical radius, while another group containing all SB galaxies have a steep r 21 gradient, decreasing by ∼20% before 40% of the optical radius, which also corresponds to the radius of the stellar bar. After this radius, these galaxies become consistent with a constant or shallow trend in r 21 . The large scale trend in r 21 can have implications for how we interpret observations made solely in the 12 CO( J = 2 − 1) line.