THE EVOLUTION OF QSO SPECTRA

We study the rest-frame UV spectra of two samples of QSOs, a low-redshift (z similar to 0.4) sample drawn from the IUE archives, and a high-redshift (z similar to 2.2), optically selected sample, the Large Bright QSO Survey (LBQS). The samples overlap substantially in their range of absolute magnitudes and radio properties, allowing us to study directly the evolution of QSO spectra between redshift z similar to 2 and the present. We find strong evidence for spectral evolution of QSOs. At high redshifts only, we see a new population of radio-quiet QSOs with unusually compact broad-emission-line regions. No significant evolution is seen in the spectra of radio-loud QSOs. We also find that Nv (124.0-nm) emission is weaker in radio-loud QSOs, and that the Baldwin effect is stronger in the red wing of C IV than in the blue.