  Intrinsic absorption is a fundamental physical property to understand the
evolution of active galactic nuclei (AGN). Here a sample of 1290 AGN, selected
in the 2-10 keV band from different flux-limited surveys with very high optical
identification completeness is studied. The AGN are grouped into two classes,
unabsorbed (type-1) and absorbed (type-2), depending on their optical
spectroscopic classification and X-ray absorption properties, using hardness
ratios. Utilizing the optical to X-ray flux ratios, a rough correction for the
~8% redshift incompleteness still present in the sample is applied. A strong
decrease of the absorbed fraction with X-ray luminosity is found. This can be
represented by an almost linear decrease from ~80% to ~20% in the luminosity
range log L_X=42-46 and is consistent with similar derivations in the optical
and MIR bands. A significant increase of the absorbed fraction with redshift is
found, which can be described by a power law with a slope ~(1+z)^{0.62+/-0.11},
saturating at a redshift of z~2. A simple power law fit ~(1+z)^{0.48+/-0.08}
over the whole redshift is also marginally consistent with the data. The
variation of the AGN absorption with luminosity and redshift is described with
higher statistical accuracy and smaller systematic errors than previous
results. The findings have important consequences for the broader context of
AGN and galaxy co-evolution. Here it is proposed that the cosmic downsizing in
the AGN population is due to two different feeding mechanisms: a fast process
of merger driven accretion at high luminosities and high redshifts versus a
slow process of gas accretion from gravitational instabilities in galactic
disks rebuilding around pre-formed bulges and black holes.
