Optical line ratio diagrams, [O III]/Hβ versus [N II]/Hα, [S II]/Hα, and [O I]/Hα (from left to right). Observations are color-coded with gray shades based on the AGN contribution to the mid-IR (5 − 40 μm), f_AGN. Different symbols indicate the classification of the targets (as labeled in the legend). The solid black curves are the criteria for separating AGN from H II regions and LINERS proposed by Kewley et al. (2006). The dotted curve is the demarcation line between AGN and H II regions from Kauffmann et al. (2003). The dashed green curves indicate AGN models from F16 (Sect. 6.1) with Z = 0.017, n_H = 10³ cm⁻³, ξ_d = 0.3, α = −1.7, and the ionization parameter log(⟨U⟩) varying from −4.5 to −1.5 (from bottom to top). In the left panel, dashed lighter and darker green curves are for AGN models of Z = 0.008 and Z = 0.04. The arrows illustrate the effect of adding to an AGN model with Z  = 0.017, log(⟨U⟩)  = −2.5, n_H = 10³ cm⁻³, ξ_d = 0.3, and α = −1.7 (black cross) a 0% to 90% fractional contribution from star formation (violet) and shocks (orange) to the total Hβ line emission. The asterisks (empty diamonds) indicate predictions of AGN+SF (AGN+shocks) models with 90% contribution to the total Hβ line from the star formation (shocks). The ionization parameter of the stellar models increases from log(⟨U^⋆⟩) = −3.0 (from the lighter to the darkest violet shade). The shock velocity increases from 200 to 1000 km s⁻¹ (from the lighter to the darkest orange shade).