  We have determined the cosmological evolution of the density of AGN and of
their Nh distribution as a function of the un-absorbed 2-10 keV luminosity up
to redshift 4. We used the HELLAS2XMM sample combined with other published
catalogs, yielding a total of 508 AGN. Our best fit is obtained with a
luminosity-dependent density evolution (LDDE) model where low luminosity
(Lx~10^43 erg s^-1) AGN peak at z~0.7, while high luminosity AGN (Lx>10^45 erg
s^-1) peak at z~2.0. A pure luminosity evolution model (PLE) can instead be
rejected. There is evidence that the fraction of absorbed (Nh>10^22 cm^-2) AGN
decreases with the intrinsic X-ray luminosity, and increases with the redshift.
Our best fit solution provides a good fit to the observed counts, the cosmic
X-ray background, and to the observed fraction of absorbed AGN as a function of
the flux in the 10^-15<S(2-10)<10^-10 erg s^-1 cm^-2 range. We find that the
absorbed, high luminosity (Lx>10^44 erg s^-1) AGN have a density of 267 deg^-2
at fluxes S(2-10)>10^-15 erg s^-1 cm^-2. Using these results, we estimate a
density of supermassive black holes in the local Universe of 3.2 (h^2_70) x
10^5 M_sol Mpc^-3, which is consistent with the recent measurements of the
black hole mass function in the local galaxies.
