LVG results for the [ CI ] line flux ratios observed in the galaxy sample. The panel contains lines of constant model flux ratio, as a function of H₂ gas kinetic temperature T_k and volume density n(H₂), for distinct C column densities N_C/dV. Curves are depicted in groups of constant line ratio [ CI ](1−0)/[ CI ](2−1) = 1 (left, bottom), 2 (middle), and 3 (right, top). Within each group of ratios, curves are drawn for column densities N(C)/dV = 0.3 × 10¹⁷ cm^-2/ km s^-1 (dotted line), 1 × 10¹⁷ cm^-2/ km s^-1 (continuous solid line), and 3 × 10¹⁷ cm^-2/ km s^-1 (dashed line), respectively.