X-RAY-VERSUS OPTICALLY SELECTED ACTIVE GALACTIC NUCLEI - A COMPARATIVE-STUDY OF THE LUMINOSITY FUNCTIONS AND EVOLUTION RATES

We compare statistical and evolutionary properties of active galactic nuclei (AGNs) and quasars selected at optical and soft X-ray wavelengths. We show that two related questions - the relationship between X-ray and optical emission and the evolution rate of X-ray-selected AGNs . probably require major revisions. New data from ROSAT deep surveys and a reanalysis of the Einstein Medium Sensitivity Survey (EMSS) indicate that the evolution rate of X-ray AGNs could have been underestimated and could actually be comparable to that of optical quasars. We also find that optical and X-ray emissions of X-ray-selected AGNs scale linearly. Adoption of our new best solutions for the optical and X-ray evolution models, and for the L(O)/L(X) dependence, leads to an impressive agreement of the luminosity functions (LFs) over wide luminosity and redshift intervals: the two wavebands appear to select essentially the same population of AGNs. On this basis, we argue that the small fraction of spurious contaminants, which allows unbiased sampling all over the luminosity-redshift plane, provides X-ray observations with considerable advantages over the usual optical selection techniques.