CO spectral line energy distributions for velocity-integrated brightness temperature (left) and velocity-integrated flux density (right), both normalized to CO(1–0), in our four selected regions (filled symbols), in the entire system (global), in the same simulation but during the starburst episode (ULIRG), in a gas-rich clumpy disk (high-redshift), and in a Milky Way-like galaxy (open symbols). The SLEDs of the most actively star-forming regions (nuclei and overlap) resemble that of the Antennae (global and ULIRG), while the arc better matches the high-redshift disk at high J_upper.