[N ii]/Hα versus [O iii]/Hβ diagnostic diagram for the 6dFGS data. The solid line is the theoretical maximum starburst limit (Kewley et al. 2001). The sources that are located above the line are classified as AGN. Galaxies that lie below the star formation demarcation dashed line (Kauffmann et al. 2003a) are classified as H ii region-like galaxies. The galaxies that lie between the two demarcation lines are on the AGN-H ii mixing sequence and are called composite galaxies. The dark cloud represents SDSS galaxies from a previous study by Kewley et al. (2006). Green circles represent galaxies that follow the narrow lines width criteria (see Sect. 5.1), and red squares represent those that do not.