UV line diagrams for the SED of POPSTAR models (Fig. 1a). A gas density of 10² cm ⁻³ is assumed. The curves present our grid of models in the range of Z = 0.01–2 Z_⊙ for log U = −3.5 (red), − 3.0 (orange), − 2.5 (yellow), − 2.0 (green), − 1.5 (cyan), − 1.0 (skyblue), and − 0.5 (blue). The metallicity of Z = 0.01, 0.1, and 1 Z_⊙ is denoted with a large filled, medium open, and small gray-filled circle, respectively, along each of the curves. The black dashed curves are the demarcations between a star-forming galaxy and AGN that will be defined in Sect. 4.1.