Log of the C⁺ column density as a function of gas density and temperature for the [C ii] intensity at the (0, 0) position of I _[C   ii]  = 7.0  ×  10^-5 erg cm^-2 s^-1 sr^-1. The bottom of the color scale (black) shows the high-temperature high density limit from Eq. (2) which yields log(N_C ⁺ )  ≥  16.65 – in the range of parameter space presented here, the limit is not reached. The column densities are for optically thin emission. Contours are plotted at column densities of log(N_C⁺)  =  17, 17.3, and 17.6. We have used Eq. (A1) from Crawford et al. (1985) assuming a critical density of 3000   cm^-3.