vs. [S ii]/[S iii] for the models. The colors code the hardness of the ionizing radiation. As there is no noticeable difference due to the morphology of the region, contrary to the case of [O ii]/[O iii] shown in Fig. 10, both morphologies are plotted here in the same figure. The blue line corresponds to our linear fit (taking only star forming regions into account, i.e., Q_{0 / 1} > 0.55; see text for the values). The red dashed line corresponds to the fit by Diaz et al. (1991).