Stacked spectra of the sources in our sample. Upper panel. Stacked spectrum (black) of all the 29 SF galaxies of the sample. Middle panel. Stacked spectrum of all the certain 4 AGNs of the sample (no X-ray detection). Lower panel. Stacked spectrum of all the possible 10 AGNs of the sample. The noise obtained from bootstrapping is shown in grey. For clarity, the Gaussian fits with 1 σ regions for the emission lines Lyα, N V, C IV, He II and C III in each stacked spectrum considered in this work are also present. Certain and possible AGN composite show clear broad nebular emission line features in their spectra, while the SF stack show much narrower lines.