Comparison of the stacked spectra, in the Hβ–[OIII] doublet lines wavelength range, in bins of [OIII]λ5007 luminosity above (left) and below (right) the line threshold, as defined in Sect. 4.2 (lower panel). Stacked spectrum in the [OIII] luminosity bin Log (L_[OIII]/erg s⁻¹) > 42.5 of the low-SF Type II AGN with superimposed the 1σ uncertainties estimated through a bootstrap resampling technique and the best-fitting one-component Gaussian curve (black dashed line). The inset shows the best-fit double-Gaussian model (magenta curve) and its line decomposition (green and blue Gaussian profiles refer to the narrow and broad best-fit components).