(Color online) Top: radial distribution of the CO and IR surface brightness in units of MJy sr^-1 for infrared data and K   km  s^-1 for CO. For the dust tracers, the surface brightness is computed both on the region covered by our CO observations (solid line) and on the whole elliptic annuli for each radius (dashed lines), showing that our CO sampled area is representative of the galaxy for each radius. Bottom: cumulative fractions of the area (solid line) and 70    μm surface brightness (dashed line) in the region covered by our CO observations (black area in the lower left diagram) compared to the whole elliptic region of radius less than R (sum of grey and black areas at a given R).