Diagnostic diagram of the L_[Cii]/L_FIR vs. L_CO/L_FIR ratios (see also caption). We superimpose our adjusted observations, assuming 70% of the total [Cii] emission arises from PDR regions (red semi-open circles), 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). Downward triangles represent upper limits. The comparative galaxy samples of Hailey-Dunsheath et al. (2010), where this figure is adapted from, are also shown with the corresponding symbols in the legend. The high-z submillimetre galaxies (blue stars) are from Gullberg et al. (2015). The grey shaded region denotes the 1σ spread in the mean observed L_[Cii]/L_CO ratio (blue dashed line).