Left column: [OIII]/Hβ vs. [NII]/Hα (top), [OIII]/Hβ vs. [OI]/Hα (centre), and [OIII]/Hβ vs. [SII]/Hα (bottom) BPT diagrams for emission lines with a 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 magenta star shows the position in the diagram of the plume of ionised gas observed at 15 kpc to the east by Gavazzi et al. (2000). 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), those in the middle left panel show the shock models of Rich et al. (2011) for increasing shock fractions (from left to right) in a twice-solar gas. Right column: map of the spaxel distribution colour-coded according to their position in the BPT diagram.