LVG results for the [ CI ]-to-¹²CO line flux ratios observed in the galaxy sample. The panels contain lines of constant flux ratio ¹²CO (4−3)/[ CI ] (1−0) (solid lines) and ¹²CO (7−6)/[ CI ] (2−1) (dashed lines) as a function of carbon and carbon monoxide column densities N_C/dV and N_CO/dV. From left to right: a. model curves at (T_k = 20 K, n = 10⁴ cm^-3) for ratios ¹²CO (4−3)/[ CI ] (1−0) = 1.1, 1.4, 2.0 and ¹²CO (7−6)/[ CI ] (2−1) = 0.5, 0.8, 1.1; b. model curves at (T_k = 25 K, n = 3 × 10⁴ cm^-3) for ratios ¹²CO (4−3)/[ CI ] (1−0) = 1.4, 2.0, 2.6 and ¹²CO (7−6)/[ CI ] (2−1) = 0.8, 1.1, 1.4; c. model curves at (T_k = 30 K, n = 10⁵ cm^-3) for ratios ¹²CO (4−3)/[ CI ] (1−0) = 2.0, 2.6, 3.2 and ¹²CO (7−6)/[ CI ] (2−1) = 1.1, 1.4, 1.7. The diagram axes are column densities for a velocity interval of 1 km s^-1.