THE PROPERTIES OF X-RAY SELECTED ACTIVE GALACTIC NUCLEI .2. A DEEPER LOOK AT THE COSMOLOGICAL EVOLUTION

Using a sample of 448 X-ray selected active galactic nuclei extracted from the Einstein Observatory Extended Medium Sensitivity Survey we have investigated in greater detail than previously done their cosmological evolution. The data have been analyzed within the framework of pure luminosity evolution (PLE) 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. Using a method that allows us to investigate luminosity evolution in redshift or luminosity shells we find evidence for luminosity-dependent luminosity evolution (LDLE) if the evolution function has the exponential form. However, the evidence for luminosity-dependent evolution becomes marginal if the data are fitted with a power-law evolution function and the simpler PLE model is, in this case, still acceptable. Similar results were obtained, in the optical domain, from the analysis of a sample of optically selected QSOs with z < 2.2 and B < 20. The EMSS AGNs X-ray number-flux relationship has been obtained. It can be described by a power-law N(> S) = K x S(-alpha) with best-fit value for the slope alpha = 1.61 +/- 0.06. It has been compared with the extragalactic ROSAT log N(> S) - log (S) recently obtained and a good agreement is found.