Context : Establishing the number of faint AGNs at z = 4 - 6 is crucial to understand their cosmological importance as main contributors to the reionization of the Universe .

Aims : In order to derive the AGN contribution to the cosmological ionizing emissivity we have selected faint AGN candidates at z > 4 in the CANDELS GOODS-South field which is one of the deepest fields with extensive multiwavelength coverage from Chandra , HST , Spitzer and various groundbased telescopes .

Methods : We have adopted a relatively novel criterion .
As a first step high redshift galaxies are selected in the NIR H band down to very faint levels ( H \leq 27 ) using reliable photometric redshifts .
This corresponds at z > 4 to a selection criterion based on the galaxy rest-frame UV flux .
AGN candidates are then picked up from this parent sample if they show X-ray fluxes above a threshold of F _ { X } \sim 1.5 \times 10 ^ { -17 } erg cm ^ { -2 } s ^ { -1 } ( 0.5-2 keV ) , correponding to a probability of spurious detections of 2 \times 10 ^ { -4 } in the deep X-ray 4 Msec Chandra image .

Results : We have found 22 AGN candidates at z > 4 and we have derived the first estimate of the UV luminosity function in the redshift interval 4 < z < 6.5 and absolute magnitude interval -22.5 \lesssim M _ { 1450 } \lesssim - 18.5 typical of local Seyfert galaxies .
The faint end of the derived luminosity function is about two/four magnitudes fainter at z \sim 4 - 6 than that derived from previous UV surveys .
We have then estimated ionizing emissivities and hydrogen photoionization rates in the same redshift interval under reasonable assumptions and after discussion of possible caveats , the most important being the large uncertainties involved in the estimate of photometric redshift for sources with featureless , almost power-law SEDs and/or low average escape fraction of ionizing photons from the AGN host galaxies .
Indeed both effects could in principle significantly reduce the estimated average volume densities and/or ionizing emissivities especially at the highest redshifts .

Conclusions : We argue that , under reasonable evaluations of possible biases , the probed AGN population can produce at z = 4 - 6.5 photoionization rates consistent with that required to keep highly ionized the intergalactic medium observed in the Lyman- \alpha forest of high redshift QSO spectra , providing an important contribution to the cosmic reionization .