Evolution of the Luminosity Function of Quasars
The luminosity function of quasars shows a strong dependence on redshift. We discuss alternate descriptions of this evolution. Density evolution is assumption-free if the density variation with red-shift is also allowed to be a function of absolute magnitude. Luminosity evolution and, in particular, pure luminosity evolution make a priori assumptions about the shape of the luminosity function. We illustrate the derivation of density evolution and its strong dependence on optical luminosity. We discuss work by Schmidt and Green (1985) on X-ray counts of quasars and the need for negative X-ray luminosity evolution to explain the counts and the low average redshift of X-ray quasars. As a consequence the quasar contribution to the 2 keV X-ray background is only around 8–13%. The evolution of active galactic nuclei of lower optical luminosity is tightly constrained by the observed X-ray background.
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