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CHAPTER 1: INTRODUCTION
mous distances, we can measure their intrinsic luminosity at different epochs of
the Universe and hence estimate how it is related to the accretion rate of matter
at those epochs. It is believed that the evolution observed in the luminosity of
AGN is directly linked to intrinsic changes in the matter inflow, which tell us
how the supermassive black hole grew and fed in the early stages of the Universe.
However, according to the unified model of AGN we have described
before, Type-2 AGN are more likely to be observed at high energies (> 2 keV)
since only hard X-rays are capable of passing through the molecular torus that
enshrouds the central engine of the AGN. Hence, this absorption has to be taken
into account in order to obtain unbiased intrinsic luminosities. In Chapter 5 we
will address this subject as well as the possible evolutionary models for AGN.
Specifically, the issues we have investigated in this thesis are:
1. To study the flux distribution of sources detected in soft and hard X-rays at
medium fluxes. This distribution depends on the cosmological properties
of the sources and can be described by an empirical model. We have used
this model to estimate the contribution of the sources to the total CXB
intensity. At what fluxes do the dominant sources of the CXB contribute
most?
2. To explore the underlying cosmic structure of the X-ray Universe. It is
known that local galaxies (redshifts up to ∼0.2) cluster forming large-scale
structures (filaments and voids). AGN are at cosmological distances and
hence they will allow us to investigate whether these structures were already
present in the early Universe and to quantify the strength of the
clustering.
3. To study the behaviour of the fraction of absorbed AGN at different X-
ray luminosities and redshifts and to model the intrinsic absorption of the
AGN as a function of these parameters. Accordingly to the unified model,
it is more probable to detect Type-2 AGN in hard X-rays since only this
radiation can pass through the molecular torus around the central black
hole. Does the fraction of obscured AGN depend on the luminosity? Were
there more absorbed AGN in the early Universe?
4. To investigate the evolution of AGN at different epochs of the Universe.
Were they more luminous in the past? Has the density of AGN changed
with time? There is evidence of this, so we intend to quantify better these
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