Predicted emission-line flux ratio of [Oiii]51.80/[Niii]57.21 as a function of gas metallicity. Blue and red lines denote the models with U = 10^-2.5 and 10^-1.5, and solid and dashed lines denote the models with n_H = 10^1.0 cm^-3 and 10^3.0 cm^-3, respectively. The flux ratio observed in M 82 and in the Antennae galaxy is shown by black horizontal line (these two galaxies show very similar [Oiii]51.80/[Niii]57.21 flux ratios; see Table 5). The x-range of this horizontal line corresponds to the inferred metallicity range for M 82 and the Antenna galaxy.