Quasars: cosmological evolution and x-ray background contribution

Abstract

The luminosity function of quasars varies with redshift or cosmic epoch. We discuss how the luminosity function and its evolution can be determined from complete samples of quasars. We first concentrate on optical survey of quasars. For quasars of medium luminosity, the co-moving space density rises very steeply with redshift. Quasars of lower luminosity exhibit a slower increase of density with redshift, resulting in luminosity-dependent evolution of the space density. We also discuss evidence for a cutoff of quasar redshift and for a possible dependence of the cutoff on luminosity. We evaluate X-ray counts of quasars and show the need for negative X-ray luminosity evolution in order to explain the counts and the low average redshifts of X-ray quasars. As a consequence, the quasar contribution to the X-ray background is lower than originally suspected. We discuss other extragalac-tic contributors to the X-ray background and conclude that they, together with the quasars, contribute about 60 percent of the observed background. About half of this is contributed by active galactic nuclei with optical luminosities below those of quasars.