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7.1. Evidence of Evolution

Given the difficulties and ambiguities in delineating the BE in QSOs at low and moderate redshift, it is not surprising that only limited attention has so far been given to the possibility that the BE evolves with time. Francis & Koratkar (1995) combined IUE data and optical spectra of LBQS quasars to study the evolution of quasar spectra with redshift over the interval from 0.4 to 2.2. Both the low- and high-redshift samples displayed a BE of comparable amplitude, as measured for C IV and Lyalpha. However, these researchers found that the UV spectra of radio-quiet quasars do evolve between redshift 2 and the present in the sense that the high-redshift quasars have a population with weak-line cores that is not seen in the IUE sample. The question of evolution of spectral properties with redshift is important for at least two reasons: 1) any evolution would affect conclusions drawn from the BE about cosmological parameters, and 2) spectral evolution could provide clues about the evolution of quasars and AGNs themselves. One difficulty in the interpretation of the findings by Francis & Koratkar rests with the IUE sample, which was not well-defined in any sense, but rather represented what was available in the archive. A similar analysis with better sample selection criteria would be preferable. This is a topic that definitely needs continued study.