Result of the stacking of SDSS-DR7 spectra with a damped Lyman-α system at an absorption redshift higher than 3.7 and a column density N(H i)  ≥  10^20.5   cm^-2. The spectra are normalized to 1 near 1280 Å (in the quasar restframe). Owing to the presence of the DLA, the flux is expected to be equal to zero at observed wavelengths below 4280 Å. Evidently this is not the case in the very blue of the spectrum (λ_obs  ≤  4000 Å) where the mean flux is increasing. Consequently, pixels at wavelengths below 4000 Å are not used in this analysis.