BPT or diagnostic diagram in the IR range, proposed to distinguish between spectral types (Fernández-Ontiveros et al. 2016). Models computed with CLOUDY v17.02 (Ferland et al. 2017) are presented as lines: black for SFG models computed from the library STARBUST99 (González Delgado & Leitherer 1999) with 12+log(O/H) = 8.69 and log(N/O) = −0.86; magenta for dwarf models computed from a starburst of ∼10⁶ yr with 12+log(O/H) = 8.0 and log(N/O) = −0.86; and blue for AGN models computed from the same SED used in this work (α_OX = −0.8 and α_UV = −1.0, see Sect. 3) with 12+log(O/H) = 8.69 and log(N/O) = −1.0. For all models, dotted lines trace models with the same electronic density (ranging 10–10⁷ cm⁻³), while dashed lines represent fixed values of the ionization parameter U.