CO line ratio maps for four selected ratios. All panels show the data convolved to 27 arcsec (indicated by circles in the lower-left corners). Only lines of sight with S/N > 5 in both lines are shown. Contours in each panel show the ¹²CO (2–1) emission at 2, 4, 6, 10, and 20 K km s^–1. A local enhancement of the line ratio in the centre of the galaxy (indicated by a plus sign) is clearly visible for ¹²CO (3–2)/(2–1) (second to left panel) and to some extent also for ¹²CO (2–0) (left and second to right panels). The C¹⁸O (2–1)/¹²CO (1–0) (right panel) is only significantly detected in the centre of the galaxy.