Collisional calculation of the ratio of the [Cii]λ 158 μm to [Oi]λ 63 μm line intensity ratio in the temperature versus density plane. The dashed lines indicate constant values of the gas thermal pressure. The observed [Cii] to [Oi]63 μm ratio in 3C 326N [Cii]/[Oi] (see text for details). For the pressures inferred in Fig. 6 and the constraint on the observed [Cii] to [Oi]63 μm ratio, the ranges of gas densities and temperatures compatible with the observations are 7 × 10²<n_H< 3 × 10³ cm^-3 and 70 <T< 100 K respectively. The grey parallelogram gives the observational constraints from Fig. 6 and the [Cii]/[Oi] ratio.