Correlation plot of I_{[ C ii ]}/I_FIR versus I_{CO (1 − 0)}/I_FIR. Edge-on model predictions for selected gas densities are plotted as lines. Dashed lines are for A_V,los = 2.5, with dark gray corresponding to a gas density of n_H = 3.0 × 10³ cm^-3, medium gray to n_H = 1.6 × 10⁴ cm^-3, and light gray to n_H = 4.0 × 10⁴ cm^-3; dotted lines are the same but for A_V,los = 5.0; the medium and light gray dash-dotted lines are for A_V,los = 0.5, with n_H = 1.6 × 10⁴ cm^-3 and n_H = 4.0 × 10⁴ cm^-3, respectively.