Colour map of the [N ii]λ6583/Hα line ratio derived from the fitting procedures described in the text, for a subset of the observed galaxies. The black solid contours shows the Hα observed intensity, starting at 0.3  ×  10^-16 erg s^-1 cm^-2, and with a step of 1  ×  10^-16 erg s^-1 cm^-2 for each contour. The blue dashed contours shows the continuum intensity at the R_C-band, starting at 0.03  ×  10^-16 erg s^-1 cm^-2, with a step of 0.1  ×  10^-16 erg s^-1 cm^-2 for each contour.