Low-J CO line ratios for 152 CO map pairs

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_-0.14_^+0.09^dex for R_21_, 0.27_-0.15_^+0.13^dex for R_31_, and 0.08_-0.09_^+0.11^dex for R32. All three line ratios anticorrelate with galactocentric radius and positively correlate with the local SFR surface density and specific SFR, 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 interstellar medium. Because the line ratios are expected to anticorrelate with the CO(1-0)-to-H2 conversion factor, {alpha}_CO_^1-0^, these results have general implications for the interpretation of CO emission from galaxies.