Emission line diagnostic diagrams. Error bars have been plotted, and an arrow added for upper or lower limits. Left panel: [N ii] diagnostic diagram. The shape of the marker indicates the final classification of nuclear activity: circle – SFN; pentagon – TO; triangle – Sy2; square – LINER, diamond – NLAGN. Filled markers correspond to galaxies classified using this diagnostic diagram, and the colour indicates the derived classification: red – AGN; green – TO; blue – SFN; cyan – TO or SFN; light green – TO or AGN. See Sect. 3 for a complete description of the classification scheme. Middle panel: [S ii] diagnostic diagram. The AGN-SFN separation curve of Kewley et al. (2001) is shown as a black solid line and the Sy2-LINER separation line of Kewley et al. (2006) as a green dashed line. Symbol shapes are the same as in the [N ii] diagram and the colours indicate the classification obtained with this diagram: red – Sy2; yellow – NLAGN; orange – LINER; and blue – SFN. Right panel: [O i] diagnostic diagram. Symbol shapes are the same as in the [N ii] diagram. Colours and separation curves are the same as in the [S ii] diagram. Empty symbols correspond to galaxies that cannot be unambiguously classified using this diagram but that were eventually classified using the other diagrams (see text).