The cosmological evolution and luminosity function of X-ray selected active galactic nuclei.

Using a sample of more than 420 X-ray selected Active Galactic Nuclei (AGNs) the authors have derived the local (z ≤ 0.1) observed X-ray Luminosity Function (XLF) and the XLF in different redshift shells up to z ≡ 2. The AGNs XLF at different redshifts gives direct evidence of cosmological evolution. The data have been analyzed within the framework of luminosity evolution models and the two most common evolutionary forms have been considered. Luminosity-dependent evolution is required if the evolution function has the exponential form, whereas the simpler pure luminosity evolution model is still acceptable if the evolution function has the power-law form. Using the whole sample of objects a de-evolved (z = 0) XLF has been derived. A good fit with a broken power law has been obtained.