Predicted [N II]_122 μm/[N II]_205 μm luminosity ratio, as a function of the gas density. The results from our Cloudy models are shown as colored symbols, with small offsets introduced for the sake of clarity. These models are color-coded by the hardness of the impinging radiation field, parameterized as the effective temperature of black bodies, T_*. We also show the predicted ratios for our analytical prescriptions as solid lines, color-coded by gas temperature, T_gas. The observed ratio is marked with green shading. The predicted ratio is practically independent of T_* or T_gas, and is solely determined by the gas density, n.