Top left panel: distribution of the [O iii] λ5007/Hβ vs. [N ii] λ6583/Hα line ratios for the ~5000 H ii ionized regions included in our sample. The colored solid lines represent the expected line ratios derived from the MAPPINGS-III ionization models, based on an instantaneous zero-age starbust model based on the PEGASE spectral energy distribution (Kewley et al. 2001; Dopita et al. 2000), and a fixed electron density of 350 cm^-3. Each orange-to-red solid line corresponds to a different metallicity, which is indicated with the corresponding label and color (ranging from 0.2 to 2.0 solar metallicities), and each blue-to-cyan solid-line corresponds to a different ionization parameter of the radiation field (ranging from log (u) = −3.7, for the dark blue, to log (u) = −2, at the cyan blue). Top right panel: similar plot, with the only difference being that now the colored solid lines represent the expected line ratios for a continuous starburst model 4 Myr since its ignition. Bottom left panel: similar plot, with the only difference being in this case that the colored solid lines and the dotted lines correspond to the expected line ratios for a photoionization model based on an instantaneous zero-age starburst, with a constant metallicity (indicated with a label and represented with a fixed color), but for two different electron densities (10 cm^-3, for the dotted line, and 350 cm^-3, solid one). Bottom-right panel: similar plot, with the only difference being that in this case the colored solid lines and the dotted lines correspond to the expected line ratios for a photoionization model based on an instantaneous zero-age starburst, with constant metallicity and a fixed electron density shown in top left panel, but for two different dust attenuations (no dust, for the solid-line, and A_V = 3 mag for the dotted one). In all the panels, the dot-dashed and dashed line represent the Kauffmann et al. (2003) and Kewley et al. (2001) demarcation curves, respectively. They are usually invoked to distinguish between classical star-forming objects (below the dashed dotted line), and AGN powered sources (above the dashed line). Regions between both lines are considered intermediate ones.