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CHAPTER 6: SUMMARY OF THE RESULTS
Finally, we have used our best-fit XLF to compute the accretion rate density
and total accreted mass onto supermassive black holes as a function of
redshift. The total black hole mass density at z = 0 predicted by our bestfit
model is in agreement with the local supermassive black hole density of
[144]. As shown by the XLF model, the high-luminosity AGN have a more
efficient growth in the early stages of the Universe and are fully formed at
z ∼ 1.5 − 2 while the less luminous AGN keep forming down to redshifts
below 1. This behaviour is also found in the ultrahard sample thus confirming
that the evolution of the XLF along cosmic time is not caused by
changes in the environment absorption but by intrinsic variations in the
accretion rate at different epochs of the Universe.
We can conclude that we have achieved to show the importance of mediumdeep
serendipitous X-ray surveys for clustering and evolutionary studies. We
have shown that these sources are essential to trace the large-scale structure of
the cosmic web and to resolve the discrete origin of the Cosmic X-ray Background
in the 0.5-10 kev energy broadband. With them, we have been able to
accurately constrain the evolutionary history of AGN thus showing that obscured
accretion power plays a key role in the understanding of the cosmic
emission and evolution of the X-ray Universe.
6.2 Future prospects
Although the results presented in this thesis have contributed substantially to
the undestanding of evolutionary and clustering properties of AGN, there are
some aspects in which would be desirable to go deeper into.
• Although we have obtained evidence that extragalactic sources at medium
fluxes detected in soft X-rays cluster strongly forming cosmological largescale
structures, we were not able to confirm this for sources detected in
hard X-rays. We can find in the literature discrepant results on this subject,
being the main reason the low density of sources per field. In an ongoing
work using a large sample of serendipitous X-ray sources described in
[151], we intend to compute the most precise angular correlation function
of extragalactic sources detected in both soft and hard X-rays to date. Additionally,
we pursue to study the possible dependence on the flux limit of
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