THE PROPERTIES OF X-RAY SELECTED ACTIVE GALACTIC NUCLEI .1. LUMINOSITY FUNCTION, COSMOLOGICAL EVOLUTION, AND CONTRIBUTION TO THE DIFFUSE-X-RAY BACKGROUND

The X-ray luminosity functions (XLF) of X-ray-selected active galactic nuclei (AGNs) and its cosmological evolution are derived and discussed. The sample utilized consists of more than 420 X-ray-selected AGNs extracted from the Einstein Observatory Extended Medium-Sensitivity Survey. The local (z < 0.1) observed XLF as well as the XLF in different redshift shells up to z approximately 2 have been derived. The local XLF shows a substantial flattening for L(x) less-than-or-equal-to 5 x 10(42) ergs s-1, while at higher luminosities it can be approximated by a power law. The AGN XLF obtained in a model-independent way at different redshifts gives direct evidence for cosmological evolution (weak for z less-than-or-equal-to 0.4 and more pronounced for higher z). The data have been analyzed within the framework of pure luminosity evolution models, and the two most common evolutionary forms L(x)(z) = L(x)(0)e(C-tau) and L(x)(z) = L(x)(0)(1 + z)C have been considered. Best-fit values of 4.18 and 2.56, respectively, have been found for the evolution parameter C. The de-evolved X-ray luminosity function has been derived. A good fit with a broken power law (with slope gamma-1 = 1.35 for L(x)(0) less-than-or-similar-to 2.5 x 10(43) ergs s-1 and slope gamma-2 = 3.05 for L(x)(0) greater-than-or-similar-to 2.5 x 10(43) ergs s-1) has been obtained. There is no evidence for a further slope change outside the range of luminosities sampled (10(42)-10(47) ergs s-1). The simultaneous determination of the cosmological evolution and of the XLF is then used to estimate the contribution of the AGNs to the extragalactic diffuse X-ray background. Using the best-fit values for the evolution of AGNs and for their volume density, it is found that they contribute approximately 40% of the 2 keV diffuse X-ray background. About 85% of this contribution comes from objects with L(x)(0) between 10(42) and 10(44) ergs s-1.