Surface mass densities of molecular hydrogen (top panel), atomic hydrogen (middle panel), and total gas (bottom panel) as a function of the surface brightness at 250 μm on a pixel-by-pixel basis. Different symbols distinguish non-deficient (red circles), slightly HI-deficient (black triangles), and HI-deficient galaxies (blue stars). The pixel size has been set to 36′′. The horizontal dotted lines show the 3σ level for the molecular and total gas (3σ = 1.15 M_⊙ pc^-2). The diagonal lines show the best linear fits. Σ_H2 in the top and bottom panels has been estimated using the constant Galactic X_CO. A typical error bar is shown in the bottom panel.