Diagnostic diagram of the observed L_[Cii]/L_FIR ratio vs. L_CO/L_FIR ratio for our sample. The observations are superimposed onto the grid of constant hydrogen nuclei density, log n (black solid lines), and FUV radiation field strength, log G₀ (black dotted lines), determined from the PDR model of Kaufman et al. (1999, 2006). The unadjusted observations (blue solid circles) are compared to the adjusted observations, as described in , where the fraction of the [Cii] emission arising from neutral gas is fixed at 70% (red semi-open circles) and 50% (red open circles). Downward triangles represent 5σ upper limits. Two sources lie above the log n = 2 contour, i.e. outside the model parameter space, and the blue arrow points towards G09.DR1.328, which remains outside the plot regardless of adjustments. The grey shaded region denotes the 1σ spread in the mean observed L_[Cii]/L_CO ratio (blue dashed line). This figure is adapted from Stacey et al. (2010a) and Hailey-Dunsheath et al. (2010).