CO SLED variation in M 51. Left: CO SLED binned by 1 kpc radial bins ranging up to 6 kpc. With the exception of the most central 1 kpc bin, we see that the CO SLED peak is already reached for J = 2 → 1. Middle: Variation with galactic environment. The spiral arm and interarm regions show a similar CO SLED shape. Right: Variation as a function of SFR surface density. We include predictions for Σ_SFR = 0.2, 0.05, and 0.02 M_⊙pc^–2 based on the model code provided in Narayanan & Krumholz (2014). We use their unresolved model, which takes different beam filling factors into account for the three CO transitions. For this analysis, we use the data at a 27 arcsec working resolution.