THE EVOLUTION RATE OF QUASARS AT VARIOUS REDSHIFTS

The evolution of optically selected quasars is usually supposed to be well described by a single constant evolution parameter, either k(L) o r k(D), depending whether we refer to luminosity or density evolution. In this paper we present a study of the variations of the evolution p arameters with redshift, for different cosmological models, in order t o probe the differential evolution with redshift. Two different quasar samples have been analyzed, the AAT Boyle's et al. and the LBQS catal ogues. Basically, these samples are divided into redshift intervals an d in each of them k(L) and k(D) are estimated by forcing that (V/V-max ) = 0.5. The dependence with respect to the cosmological parameters is small. Both AAT and LBQS show roughly the same tendencies. LBQS, howe ver, shows strong fluctuations, whose origin is not statistical but ra ther due to the selection criteria. A discussion on selection techniqu es, biases and binning effects explains the differences between these results. We finally conclude that the evolution parameter is almost co nstant in the redshift range 0.7 less than or equal to z less than or equal to 1.7, at least within 2 sigma, while it decreases slightly aft erwards. Results depend on the binning chosen (but not in a very signi ficant way). The method has been tested with Monte-Carlo simulated cat alogues in order to give a better understanding of the results coming from the real catalogues. A correlation between k(L) (k(D)) and (V/V-m ax) is also derived and is used for the calculation of the error bars on the evolution parameter.