Optical selection effects that bias quasar evolution studies

Abstract

A study of systematic biases in optical observations of quasars is described. A numerical model of the optical biases was constructed to study the effect of systematic biases on quasar discoveries and on evolution laws derived from quasar counts in 'complete' surveys. It is shown that the excess of quasars found at redshifts of about 2.5 is largely a result of photometric errors and the unrecognized contribution of strong UV emission lines to the blue magnitude of high-redshift quasars. Reexamination of the quasars brighter than blue magnitude = -24 in the Palomar Bright Quasar Survey (BQS) showed that for the brightest quasars in the survey, the data are consistent with no evolution. A comparison of BQS quasars with the brightest quasars of the CTIO Schmidt Telescope Survey (Osmer and Smith, 1980) indicated that the brighter CTIO survey quasars have much stronger C IV lines (lambda = 1548) than the brightest BQS quasars. The strong emission lines in the CTIO survey quasars are understood as a consequence of the general correlation between absolute quasar luminosity and C IV (lambda = 1548) equivalent width.