Column density of CO-dark H₂ gas for all CO sources as a function of the total hydrogen column density N_tot = N(H i) + 2N(H₂). The curves are N(CO-dark H₂) calculated from the models of Visser et al. (2009) as discussed in the text. The central curve (dashed line) is the 100-point means of their model shown in the right panel of their Fig. 7. The top curve (dotted line) is the lower limit on the solutions for N(CO), which yields a maximum for N(CO-dark H₂), and the bottom curve (solid line) is the upper limit on N(CO), which yields a minimum for N(CO-dark H₂), as a function of N(H₂) for the range of parameter space used in their models.