BPT diagrams depicting the area covered by AGN and SF models with solar abundances for −3.5 ≤ log U ≤ −1.5, shown with purple and red contours, with 1 ≤ log n_H ≤ 3.5 and, 1 ≤ log n_H ≤ 3 respectively. The solid contour lines map regions containing 10%, 50%, and 90% of the models with ζ_CR = 10⁻¹² s⁻¹ in the [N II] BPT diagram, and 10⁻¹³ s⁻¹ in both the [S II] and the [O I] panels. The gray hatched area stands for AGN models with ζ_CR = 10⁻¹⁶ s⁻¹, of the order of the average Galactic CR background (Indriolo et al. 2007). Cyan squares represent line ratios measured for the regions selected in Centaurus A, green circles for regions in NGC 1068, magenta thin diamonds for regions in NGC 253, and the red diamond correspond to the photoionization-dominated Seyfert 2 nucleus in NGC 1320. The Kewley, Kauffmann, and Schawinski lines correspond to the solid dark gray, the solid medium gray, and dashed light gray lines, respectively.