Radial distribution of the CO-derived H₂ (red) and H i (green) mass surface density in M_⊙ pc^-2. The mass surface density is corrected for inclination and includes helium. The red dots correspond to an exponential fit to the first 7 kpc, yielding an exponential length of 2.1 kpc. The blue dots represent two partial fits: one for the inner galaxy (out to 3.5 kpc), and one for the outer galaxy (3.5 kpc <R< 7.0 kpc). Statistical error bars are derived from the rms scatter over the rings, as described in the 2nd paragraph of Sect. 3.3.