Face-on optical depth in the FUV band versus the total gas mass surface density in units of M_⊙ pc^-2 for a slab geometry, and assuming a metallicity-independent X_CO factor. The blue curves represent the best fit for the full sample whereas the black curves represent the best fit for individual galaxies for which the equation is given at the top of each panel. The uncertainties on τ_FUV are indicated by the error bar at the bottom right of each panel.