Left: metallicity-ionization parameter grid from the MAPPINGS V ionization model. The red points are the ones falling outside of the SF region in the third BPT diagram. The thick lines represent the grid for a constant metallicity and the thin lines for a constant ionizing parameter. The dimensionless ionizing parameter goes from log(U) = −3.5 to log(U) = −2. Right: spatial distribution of the logarithmic values of the [OIII]/[SII] ratio, used as a proxy for the ionization parameter. We see an outside-in gradient showing that the most ionized regions are located in the centre of the galaxy. The field of view is the same as in Fig. 3.