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 Ly
. 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.