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The study of the space density of bright AGNs at $z>4$ has been subject toextensive effort given its importance for the estimate of the cosmologicalionizing emissivity and growth of supermassive black holes. In this context wehave recently derived high space densities of AGNs at $z\sim 4$ and$-25<M_{1450}<-23$ in the COSMOS field from a spectroscopically completesample. In the present paper we attempt to extend the knowledge of the AGNspace density at fainter magnitudes ($-22.5<M_{1450}<-18.5$) in the $4<z<6.1$redshift interval by means of a multiwavelength sample of galaxies in theCANDELS GOODS-South, GOODS-North and EGS fields. We use an updated criterion toextract faint AGNs from a population of NIR (rest-frame UV) selected galaxiesat photometric $z>4$ showing X-ray detection in deep Chandra images availablefor the three CANDELS fields. We have collected a photometric sample of 32 AGNcandidates in the selected redshift interval, six of which having spectroscopicredshifts. Including our COSMOS sample as well as other bright QSO samplesallows a first guess on the shape of the UV luminosity function at $z\sim 4.5$.The resulting emissivity and photoionization rate appear consistent with thatderived from the photoionization level of the intergalactic medium at $z\sim4.5$. An extrapolation to $z\sim 5.6$ suggests an important AGN contribution tothe IGM ionization if there are no significant changes in the shape of the UVluminosity function.

The space densities and emissivities of AGNs at $z> 4$