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 wavelengt hs. We show that two related questions - the relationship between X-ra y and optical emission and the evolution rate of X-ray-selected AGNs . probably require major revisions. New data from ROSAT deep surveys an d a reanalysis of the Einstein Medium Sensitivity Survey (EMSS) indica te that the evolution rate of X-ray AGNs could have been underestimate d 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 li nearly. Adoption of our new best solutions for the optical and X-ray e volution models, and for the L(O)/L(X) dependence, leads to an impress ive agreement of the luminosity functions (LFs) over wide luminosity a nd redshift intervals: the two wavebands appear to select essentially the same population of AGNs. On this basis, we argue that the small fr action of spurious contaminants, which allows unbiased sampling all ov er the luminosity-redshift plane, provides X-ray observations with con siderable advantages over the usual optical selection techniques.