  We compute the effect of the galactic absorption on AGN emission in a
cosmological context by including a physical model for AGN feeding and feedback
in a semi-analytic model of galaxy formation. This is based on galaxy
interactions as triggers for AGN accretion, and on expanding blast waves as a
mechanism to propagate outwards the AGN energy injected into the interstellar
medium at the center of galaxies. We first test our model against the observed
number density of AGNs with different intrinsic luminosity as a function of
redshift. The model yields a ''downsizing'' behavior in close agreement with
the observed one for z<2. At higher redshifts, the model predicts an overall
abundance of AGNs (including Compton-thick sources) larger than the observed
Compton-thin sources by a factor around 2 for z>2 and L_X < 10^{44} erg/s.
Thus, we expect that at such luminosities and redshifts about 1/2 of the total
AGN population is contributed by Compton-thick sources. We then investigate the
dependence of the absorbing column density N_H associated to cold galactic gas
(and responsible for the Compton-thin component of the overall obscuration) on
the AGN luminosity and redshift. We find that the absorbed fraction of AGNs
with N_H>10^{22} cm^{-2} decreases with luminosity for z<1; in addition, the
total (integrated over luminosity) absorbed fraction increases with redshift up
to z around 2, and saturates to the value around 0.8 at higher redshifts.
Finally, we predict the luminosity dependence of the absorbed fraction of AGNs
with L_X< 3 10^{44} erg/s to weaken with increasing redshift. We compare our
results with recent observations, and discuss their implications in the context
of cosmological models of galaxy formation.
