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CHAPTER 5: LUMINOSITY FUNCTION OF XMS SOURCES
with the results of [224], [130] or [211]. Our best-fit model shows weaker evolution
of the AGN below the cut-off redshift than in these works albeit with
smaller error bars. In the ultrahard band, we have also calculated the intrinsic
XLF finding similar results as in [48] but with our best-fit parameters much
more constrained. The results in this band show that ultrahard AGN present a
significantly stronger evolution below the cut-off redshift than those detected at
softer energies. This could be due to the lack of complementary deep surveys
(which probe intrinsically faint sources) in the ultrahard band. If there exists
a dependency of the evolution rate on the X-ray luminosity ([157], [104]) we
could be biased towards high-luminosity objects thus measuring higher evolution
rates. In all three bands, the high-luminosity AGN (log L X > 44) are
formed before than the low-luminosity ones (log L X < 44), reaching the former
a maximum in density at redshift z ∼ 1.5 whereas the comoving density of the
latter peak at z ∼ 0.7.
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 best-fit 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.
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