Left: [OIII]/Hβ vs. [NII]/Hα (upper) and [OIII]/Hβ vs. [SII]/Hα (lower) BPT diagrams for emission lines with an S/N > 5. The dashed curves separate AGN from HII regions (Kauffmann et al. 2003; Kewley et al. 2001, 2006). Data are colour coded according to their minimum distance from these curves. The black and grey crosses indicate the typical error on the data for lines with S/N ≃ 15 and S/N ≃ 5, respectively. The grey contours show the distribution of a random sample of nuclear spectra of SDSS galaxies in the redshift range 0.01–0.1 and stellar masses 10⁹ ≤ M_star ≤ 10¹¹ M_⊙. The thick solid lines in the upper left panel show three different photo-ionisation models at different metallicities (0.2, 0.4, 1 Z_⊙; Kewley et al. 2001). Right: map of the spaxel distribution colour-coded according to their position in the BPT diagram. The cross, star, and filled circle show the position of the photometric, stellar and gas kinematical centres, respectively. The black rectangle indicates the front region of the galaxy (see Fig. 4).