Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA

Abstract

We measure the low-J CO line ratio R 21 ≡ CO (2−1)/CO (1−0), R 32 ≡ CO (3−2)/CO (2−1), and R 31 ≡ CO (3−2)/CO (1−0) using whole-disk CO maps of nearby galaxies. We draw CO (2−1) from PHANGS-ALMA, HERACLES, and follow-up IRAM surveys; CO (1−0) from COMING and the Nobeyama CO Atlas of Nearby Spiral Galaxies; and CO (3−2) from the JCMT NGLS and APEX LASMA mapping. Altogether this yields 76, 47, and 29 maps of R 21 , R 32 , and R 31 at 20 ∼ 1.3 kpc resolution, covering 43, 34, and 20 galaxies. Disk galaxies with high stellar mass, log(M /M ) = 10.25−11 and star formation rate, SFR = 1−5 M yr −1 , dominate the sample. We find galaxy-integrated mean values and 16%−84% range of R 21 = 0.65 (0.50−0.83), R 32 = 0.50 (0.23−0.59), and R 31 = 0.31 (0.20−0.42). We identify weak trends relating galaxy-integrated line ratios to properties expected to correlate with excitation, including SFR/M and SFR/L CO . Within galaxies, we measure central enhancements with respect to the galaxy-averaged value of ∼0.18 +0.09 −0.14 dex for R 21 , 0.27 +0.13 −0.15 dex for R 31 , and 0.08 +0.11 −0.09 dex for R 32 . All three line ratios anti-correlate with galactocentric radius and positively correlate with the local star formation rate surface density and specific star formation rate, and we provide approximate fits to these relations. The observed ratios can be reasonably reproduced by models with low temperature, moderate opacity, and moderate densities, in good agree-

The Gas-Star Formation Cycle in Nearby Star-forming Galaxies II. Resolved Distributions of CO and H$α$ Emission for 49 PHANGS Galaxies

“…We do not account for galaxy to galaxy variations in R 21 . Recent studies of nearby galaxies show that R 21 for individual galaxies is around 0.5 to 0.7, and the typical scatter is ∼ 0.1 dex within individual galaxies (e.g., Yajima et al 2021;den Brok et al 2021;Leroy et al 2021c). Physically, R 21 may increase with SFR and gas density because the higher-J transition becomes brighter when gas is warm and/or dense (e.g., Sakamoto et al 1994Sakamoto et al , 1997Yajima et al 2021;den Brok et al 2021;Leroy et al 2021c).…”

“…Recent studies of nearby galaxies show that R 21 for individual galaxies is around 0.5 to 0.7, and the typical scatter is ∼ 0.1 dex within individual galaxies (e.g., Yajima et al 2021;den Brok et al 2021;Leroy et al 2021c). Physically, R 21 may increase with SFR and gas density because the higher-J transition becomes brighter when gas is warm and/or dense (e.g., Sakamoto et al 1994Sakamoto et al , 1997Yajima et al 2021;den Brok et al 2021;Leroy et al 2021c). In other words, assuming a single R 21 may result in overestimation of Σ H2 in overlap regions (given that CO and Hα emission are likely to co-exist in high-SFR and/or high-density regions).…”

“…Since α CO is defined for the 12 CO(J = 1 → 0) transition, we apply a constant 12 CO(J = 2 → 1) to 12 CO(J = 1 → 0) brightness temperature ratio of R 21 = 0.65. We do not account for galaxy to galaxy (and also inside a galaxy) variations in this ratio, which are typically ∼ 0.1 dex (Leroy et al 2013;Yajima et al 2021;den Brok et al 2021;Leroy et al 2021c). We test our results against using a constant Galactic α CO and discuss the choice of R 21 in Appendix A.1 and A.2, respectively.…”

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).XII. Ionised gas emission in the inner regions of lenticular galaxies

“…These dust features seen in the Balmer decrement correspond fairly well to those observed in absorption in the VESTIGE NB images. Comparable Notes: M(H 2 ) are derived assuming a standard CO-to-H 2 conversion factor X CO = 2.3 × 10 20 cm −2 /(K km s −1 ) (Strong et al 1988) and an intensity ratio CO(2-1)/CO(1-0) = R 21 = 0.65 and CO(3-2)/CO(1-0) = R 31 = 0.31 (Leroy et al 2021).…”

“…ALMA data for two galaxies are in the 12 CO(2-1) line and for one in the 12 CO(3-2) line. Using the same procedure applied to the CARMA data, and assuming the ratios 12 CO(2-1)/ 12 CO(1-0) = 0.65 0.83 0.50 and 12 CO(3-2)/ 12 CO(1-0) = 0.31 0.42 0.20 from Leroy et al (2021), quite consistent with previous values (e.g. Leroy et al 2009, Wilson et al 2012, we estimate total molecular hydrogen masses from the ALMA data.…”