BPT diagrams for the two narrow components with models of AGN photoionization with solar metallicity (upper panels) and twice the solar metallicity (lower panels). The models have a gas density n_e = 10³ cm⁻³, the dashed lines indicate models with constant photoionization parameter logU (from −4 to 0, from bottom to top ), and the solid lines refer to models with constant spectral index α (from −2 to −1.2, from left to right). The solid line in all the panels is the Kewley et al. (2001) maximum starburst line. The dashed line in the left panels is the semi-empirical Kauffmann et al. (2003) line. The dashed line in the central and right panels is the empirical Kewley et al. (2006) line separating Seyfert galaxies from LINERS.