Spatial and radial variation of the CO line ratio. Top row shows the R₂₁ line ratio while the central row shows the R_13/12 line ratio. The maps (left top and middle panels) show the spatial distribution of the line ratios. The colored points show sightlines with 5σ in both integrated intensities. The 5σ contour of the ¹²CO(1–0) integrated intensity is shown by the solid contour. The dashed circles indicate the radial bins used for the stacking. The radial plots (right panels) show the radial trends of the line ratios. The panels in the bottom row show trends of the ¹²CO(2–1)/(1–0) (R₂₁) ratio (left) and the ¹³CO/¹²CO(1–0) ratio (R_13/12) (right) with the SFR surface density. The ratio derived by the stacked line brightness is indicated by the larger blue or green symbols (see Appendix C for a brief description of the stacking technique). We note that because the S/N¹³CO(1–0) is significantly lower than for ¹²CO(1–0), and more lines of sights do not show a significant detection, the stacked points yield a lower R_13/12. Triangles indicate 3σ upper limits. The uncertainty of the points is indicated, but it is generally smaller than the point size. The censored region applying to the individual lines of sight is shown by the red (1σ) shaded region. The region indicates where, due to the lower average sensitivity of one observation set, we do not expect to find significantly detected data points (i.e., the region where only one dataset will be significantly detected).