Dust (green triangles), atomic gas (red stars), and molecular gas (blue circles) surface mass densities as a function of R/R₂₅ for the extended sample. The molecular component has been estimated using the Galactic X_CO (solid line) and the radially varying X_CO (Eq. (1); dashed line). The total gas mass surface density (for the Galactic X_CO only), is indicated with black diamonds. The horizontal dotted line indicates Σ = 9 M_⊙ pc^-2.