Is C IV lambda 1548 a standard cosmic candle

It is difficult to observe emission lines of small equivalent width in intrinsically faint QSOs. This selection effect biasses the slope of the log (intrinsic continuum luminosity): log (C IV equivalent width) diagram for samples of QSOs such as those of Baldwin and those taken from slitless spectrum surveys. There is no such bias for QSO samples drawn from radio surveys, and such data suggest that the total flux of the C IV line may itself be a good standard candle. The best way to determine q0 from QSO samples may be to look at the upper envelope of the msub(CIV)-z relationship. (author)