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

We present ¹²CO(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 ¹²CO(J = 2 − 1)/¹²CO(J = 1 − 0) line ratio r₂₁ are investigated. The luminosity-weighted r₂₁ 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 r₂₁ is systematically higher in outliers compared to the disk. We compare r₂₁ between SA, SAB, and SB galaxies, and find no significant difference in the average r₂₁ depending on the presence of galactic bars. We find, however, that the radial gradient in r₂₁ is bimodal, where a group containing all SA galaxies prefer constant or very shallow r₂₁ gradients out to 40% of the optical radius, while another group containing all SB galaxies have a steep r₂₁ 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 r₂₁. The large scale trend in r₂₁ can have implications for how we interpret observations made solely in the ¹²CO(J = 2 − 1) line.