Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA
  We measure the low-J CO line ratio R21=CO(2-1)/CO(1-0), R32=CO(3-2)/CO(2-1),
and R31 = 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 R21, R32, and R31 at 20" \sim 1.3 kpc resolution, covering 43, 34, and
20 galaxies. Disk galaxies with high stellar mass, log10 M_* [Msun]=10.25-11
and star formation rate, SFR=1-5 Msun/yr, dominate the sample. We find
galaxy-integrated mean values and 16%-84% range of R21 = 0.65 (0.50-0.83),
R32=0.50 (0.23-0.59), and R31=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 \sim
0.18^{+0.09}_{-0.14} dex for R21, 0.27^{+0.13}_{-0.15} dex for R31, and
0.08^{+0.11}_{-0.09} dex for R32. 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 agreement with expectations for the cold ISM. Because the
line ratios are expected to anti-correlate with the CO(1-0)-to-H_2 conversion
factor, alphaCO^(1-0), these results have general implications for the
interpretation of CO emission from galaxies.
