ALMA FACTS. II. Large Scale Variations in the 12CO(J = 2 – 1) to 12CO(J = 1 – 0) Line Ratio in Nearby Galaxies
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 investig...
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| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal |
| Subjects: | |
| Online Access: | https://doi.org/10.3847/1538-4357/ada6b5 |
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| Summary: | 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. |
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| ISSN: | 1538-4357 |