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I will present how using what we know about the
AGN population, mainly from X-ray surveys, we can explain the
observed properties of the X-ray background and at the same time
study the AGN evolution. In particular, I will focus on the number
of obscured AGN and its possible dependence on parameters like
luminosity or redshift, contrary to the expectation of the simplest
unification ideas.
Using the largest sample of x-ray selected AGN to date we found that
the relative number of obscured AGN decreases with increasing
luminosity and increases with redshift. I will present observational
evidence, mainly using SDSS quasars, showing that the dependence of
the obscured AGN fraction on luminosity can be due to a change in
the torus opening angle.
Finally, I will present the first results from a survey at very high
energies with INTEGRAL designed to find the most obscured AGN, and
show how we can construct for the first time a complete sample of
the AGN population, including the elusive population of Compton
thick AGN, and link it to the observed properties of the
supermassive black holes in the local Universe. Using this mostly
complete sample, we found that the spatial density of heavily
obscured supermassive black holes at z=0 is lower than previously
expected. |
