CO(1–0) vs. [Cii] excitation temperatures determined via the ratio of the source functions ()) assuming various line opacities, as specified in the legend, and adopting the observed L_[Cii]/L_CO ratio of 3500. The light and dark grey shaded regions denote denotes the 1σ spread in the mean observed ratio, whereas the red shaded region represents the CO excitation temperatures of 20 K <T_ex,CO(1−0)< 55 K. Combined with the results of PDR modelling and previous observational evidence in the literature (), the best scenario to explain the observed L_[Cii]/L_CO ratios is gas where T_{ex, [Cii]}>T_ex,CO(1−0) and with [Cii] is optically thin and the CO is optically thick (τ_[Cii] = 0.1, τ_CO(1−0) = 1; black dotted line).