Left panels: spatially resolved [S II]- and [N II]- BPT diagrams for MR 2251−178. Each dot corresponds to a single Voronoi bin. Red (blue) dots represent regions whose excitation is dominated by star formation (AGN), green dots are used for composite (in the [N II]-BPT) or LI(N)ER (in the [S II]-BPT) excitation regimes. Solid and dashed black lines show the separation between the different regimes, from Kewley et al. (2001, 2006) and Kauffmann et al. (2003). Only lines with S/N > 2 are used. Right panels: [S II]- and [N II]- BPT maps, colour-coded according to the dominant excitation regime. Darker colours indicate higher [S II] (top panel) or [N II] (bottom panel) intensities. Iso-intensity contours show the integrated [O III] line, smoothed to a FHWM resolution of 0.25″. The ionisation of the nebula is vastly dominated by the AGN.