Red, green, and blue (RGB) image of M 51 with ¹²CO emission, a colour HST plate composed of B, V, and I filter images taken from Mutchler et al. (2005). Overlaid as contours are the IRAM 30 m ¹²CO (2–1) integrated line brightness temperatures (at a resolution of 13 arcsec, indicated by the circle in the lower-left corner). The contours show signal-to-noise levels from 8 to 70, from blue to brown. The beam size of the ¹²CO (2–1) observations is indicated at the bottom left of the figure. The dashed region shows the field-of-view of the IRAM 30 m observations.