A STRONG DEPENDENCE OF THE NARROW CIV ABSORPTION-LINE DENSITY ON THE QUASAR EMISSION REDSHIFT

The motivation for our work was to search for indications of gravitati onal lensing in samples of narrow metal absorption line systems observ ed in quasar spectra. We here discuss statistical properties of highly displaced CIV systems, taken from the line lists of five surveys foun d in the literature. In the combined sample, the number of systems per unit redshift range is strongly dependent on the quasar emission reds hift z(e); most significant is a ''break'' at z(e) congruent-to 2.8. T he only plausible explanation is a strong selection effect varying wit h z(e); for z(e) greater-than-or-equal-to 2.8, most quasars are select ed with respect to the detection of Ly alpha emission in objective pri sm surveys. The negative redshift evolution of the absorbers, being pr esent for absorption redshifts z(e) greater-than-or-similar-to 1.8, is possibly nothing but a reflection of this selection effect. We conclu de that our result makes the association hypothesis for at least a lar ge fraction of the absorbers more probable. Due to the non-uniformity of the yet observed quasar samples, it is not possible to test as to w hether an amplification bias by gravitational lensing contributes to t he z(e)-dependence of the absorption line density.