Parameter space n_H, U, Z. Data points in 3D space are elements in the grid of the parameter space selected for not being different from by more than δχ² ≈ 1, also satisfying the condition that the three intensity ratios used for the computation of the χ² individually agree with the ratios predicted within the errors at 1σ confidence level. Data points were computed from the emission line ratios measured for the BC and referring to the median values in Table 6. The individual contour line was smoothed with a Gaussian kernel.