Emission line ratio diagrams for our models (log U and L_X/SFR) for different SEDs compared to the observational sample. The left column shows high-ionisation emission line diagrams (see Stasińska et al. 2015), while the right column shows low-ionisation diagrams (BPT diagram; e.g., Kewley et al. 2006; Baldwin et al. 1981). The points within a same model show the outputs obtained for the base models and different ionisation parameters. The shaded and coloured areas show the range of emission line ratios covered by the DIS, BMC, and KAA models with varying X-ray luminosities and an ionisation parameter. The lines show the results for the same ionisation parameter for each model (from top to bottom, log U = −1.5, −2.0, −2.5, −3.0, −3.5) and increasing X-ray emission from left to right. For the stellar (STE) and the LEB SED, the X-ray luminosity is constant by construction (and L_X = 0 for STE). The observational sample is shown by the grey symbols, where dots show the He II emitters and crosses the sources where He II is not detected. In approximately two-thirds of the galaxies, HeII emission is not detected with relative intensities below I(4686)/I(Hβ) ∼ (2−3) × 10⁻³.