Space distribution and luminosity function of quasars

Abstract

We discuss the evolution of the luminosity function of quasars in terms of either density evolution or luminosity evolution. The observations appear to support pure luminosity evolution in which the entire luminosity function shifts in the luminosity coordinate as a function of cosmic time. The simplest interpretation is that all quasars were formed some 10 billion years ago and that all suffer the same luminosity decrease as the time goes on. This picture encounters major difficulties in that it predicts (a) present quasar masses that are 100 times as large as those derived from the physics and dynamics of the broad-line region, and (b) a contribution by Seyfert galaxies to the 2 keV X-ray background that is considerably in excess of that observed. The luminosity function of quasars is of fundamental importance in the interpretation of quasar counts as a function of optical magnitude and redshift. The shape of the luminosity function and its dependence on redshift potentially convey information about the energy mechanism of quasars and its dependence on environment. Depending on the life time of the quasar phase, the luminosity function can be used to estimate the space density of dead quasars or black holes, presumably located in the nuclei of galaxies. Although we will discuss only the optical luminosity function, the general luminosity function of quasars is also a function of radio and X-ray luminosity, and possibly other properties related to the environment.

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