CO SLEDs using a two-component description of the ISM in which one component is represented by PDRs scaled to log(G₀) = 0.0 (dashed blue line) and the other is either represented by PDRs scaled to log(G₀) = 2.0, 3.0, and 4.0 or an XDR at distances from the line emitting region of 1.0 kpc and 1.5 kpc (dashed red line). The solid black line shows the best-fit composite model and the lighter gray lines show other models within 1σ. We note that although the best-fit models appear to have been normalized to the CO(12–11) line, no such normalization was performed.