Radial profiles of the molecular fraction f_mol for the local spiral galaxies. The solid line shows this model and the dashed line shows f_mol = t_ff/tmol/(1 + t_ff/t_mol), where the free-fall and molecule-formation times are those of self-gravitating clouds (Vollmer & Leroy 2011). The profile for each galaxy is shown in a different color. The observed relation R_mol = 10.6exp(−R/ 0.21R₂₅) (Leroy et al. 2008) is shown as a thick dashed line.