We investigate the population of high-redshift ( 3 \leq z < 6 ) AGN selected in the two deepest X-ray surveys , the 7 Ms Chandra Deep Field-South and 2 Ms Chandra Deep Field-North .
Their outstanding sensitivity and spectral characterization of faint sources allow us to focus on the sub- L _ { * } regime ( log L _ { \mathrm { X } } \lesssim 44 ) , poorly sampled by previous works using shallower data , and the obscured population .
Taking fully into account the individual photometric-redshift probability distribution functions , the final sample consists of \approx 102 X-ray selected AGN at 3 \leq z < 6 .
The fraction of AGN obscured by column densities log N _ { \mathrm { H } } > 23 is \sim 0.6 - 0.8 , once incompleteness effects are taken into account , with no strong dependence on redshift or luminosity .
We derived the high-redshift AGN number counts down to F _ { \mathrm { 0.5 - 2 keV } } = 7 \times 10 ^ { -18 } \mathrm { erg cm ^ { -2 } s ^ { -1 } } , extending previous results to fainter fluxes , especially at z > 4 .
We put the tightest constraints to date on the low-luminosity end of AGN luminosity function at high redshift .
The space-density , in particular , declines at z > 3 at all luminosities , with only a marginally steeper slope for low-luminosity AGN .
By comparing the evolution of the AGN and galaxy densities , we suggest that such a decline at high luminosities is mainly driven by the underlying galaxy population , while at low luminosities there are hints of an intrinsic evolution of the parameters driving nuclear activity .
Also , the black-hole accretion rate density and star-formation rate density , which are usually found to evolve similarly at z \lesssim 3 , appear to diverge at higher redshifts .