2D diagnostic diagarms. Left: [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 an S/N > 5 in the galaxy NGC 4469. 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 photoionisation models at different metallicities (0.2, 0.4, and 1 Z_⊙; Kewley et al. 2001). The lines 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: map of the spaxel distribution colour-coded according to their position in the BPT diagram. The black and white crosses indicate the position of the photometric centre of the galaxy.