Upper panel: model Hi radial profiles of the local spiral galaxies. The radial distance is normalized by the stellar scale length l_∗. The middle panel shows model Hi including UV ionization with a constant Hi volume density, while the lower panel shows THINGS radial Hi profiles from Leroy et al. (2008) where we have assumed R₂₅ = 4.5 × l_∗. The atomic gas surface density saturates at ~10 M_⊙ pc^-2 (dotted line).