Mean (black line) and median (red line) stacked detection maps calculated from the set of spectra reported in Fig. D.1. The zoomed region including C IVλ1548, 1550, He IIλ1640, O III]λ1661, 1666, and C III]λλ1907, 1909 (from left to right) is shown. Top panel: all sources are included (14 entries), while in the bottom panel source 14 is excluded, that is the source showing prominent nebular high-ionization lines (see Fig. C.1, spectrum in blue; see also Fig. D.1). The median stacks (red lines) are rigidly redshifted by a fixed quantity for illustrative purposes only.