LVG results for the [ CI ]-to-¹³CO line flux ratios observed in the galaxy sample. The panels contain lines of constant flux ratio [ CI ] (1−0)/¹³CO (2−1) as a function of carbon and carbon monoxide column densities N_C/dV and N_13CO/dV. From left to right: a. model curves at (T_k = 20 K, n = 10⁴ cm^-3) for ratios [ CI ] (2−1)/(1−0) = 1, 3, 10, 30 (thick line), 100, and 300 increasing from right to left; b. as panel a., but for (T_k = 25 K, n = 3 × 10⁴ cm^-3); c. as panel a., but for (T_k = 30 K, n = 10⁵ cm^-3). The diagram axes are column densities for a velocity interval of 1 km s^-1.