On the cosmological evolution of the X-ray emission from quasars

Abstract

The average dependence of the ratio of X-ray luminosity to optical luminosity as a function of redshift and optical luminosity for quasars is derived, and it is found that the explicit dependence of this ratio is predominantly on optical luminosity. For a wide class of models for the cosmological evolution of quasars, the results imply that the evolution of the X-ray luminosity function is weaker than the evolution of the optical luminosity function. Implications for physical models of quasars and for global properties of the quasar population are indicated.