The Luminosity Function of Quasars and its Evolution: a Comparison of Optically Selected Quasars and Quasars Found in Radio Catalogs

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Abstract

The luminosity function of quasars and its evolution are discussed, based on comparison of available data on optically selected quasars and quasars found in radio catalogs. It is assumed that the redshift of quasars is cosmological, and the results are expressed in the framework of the A = 0, q0 = 1 cosmological model. The predictions of various density evolution laws are compared with observations of an optically selected sample of quasars and quasar samples from radio catalogs. The differences between the optical luminosity functions, the redshift distributions, and the ratios of radio to optical luminosity of optically selected quasars and radio quasars rule out luminosity functions where there is complete absence of correlation between radio and optical luminosities. These differences also imply that Schmidt's luminosity function, where there exists a statistical correlation between radio and optical luminosities, although possibly correct for highredshift objects, disagrees with observation at low redshifts. These differences can be accounted for by postulating the existence of two classes (I and II) of objects. The class I nonevolving objects dominate the optically selected samples at low redshifts, have low optical luminosities of about 1022 W Hz-1 (at 2500 A), and are absent from radio samples because of their relative low ratios of radio to optical luminosity. The class II objects are the quasars found in radio catalogs, show strong evolution, have optical luminosities of 1023 to 1024 W Hz-', and dominate the optically selected samples at large redshifts. Possible relations between the two classes are also discussed, and tests for the validity of this picture are presented. Subject headings: cosmology - luminosities - quasi-stellar sources or objects - radio sources

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