The Properties of X-Ray-selected Active Galactic Nuclei. I. Luminosity Function, Cosmological Evolution, and Contribution to the Diffuse X-Ray Background
Abstract
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 ~ 2 have been derived. The local XLF shows a substantial flattening for L_x_ <= 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 <= 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^cτ^ 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 γ_1_ = 1.35 for L_x_(0) <~ 2.5 x 10^43^ ergs s^-1^ and slope γ_2_ = 3.05 for L_x_(0) >~ 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 ~ 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^.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 1991
- DOI:
- 10.1086/170102
- Bibcode:
- 1991ApJ...374..117M
- Keywords:
-
- Active Galactic Nuclei;
- Cosmology;
- Galactic Evolution;
- Luminosity;
- X Ray Astronomy;
- Astronomical Spectroscopy;
- Background Radiation;
- Heao 2;
- Astrophysics;
- GALAXIES: NUCLEI;
- GALAXIES: X-RAYS;
- LUMINOSITY FUNCTION;
- X-RAYS: SOURCES