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We have evaluated the contribution of the AGN population to the ionizationhistory of the Universe based on a semi-analytic model of galaxy formation andevolution in the CDM cosmological scenario. The model connects the growth ofblack holes and of the ensuing AGN activity to galaxy interactions. In themodel we have included a self consistent physical description of the escape ofionizing UV photons; this is based on the blast-wave model for the AGN feedbackwe developed in a previous paper to explain the distribution of hydrogen columndensities in AGNs of various redshifts and luminosities, due to absorption bythe host galaxy gas. The model predicts UV luminosity functions for AGNs whichare in good agreement with those derived from the observations especially atlow and intermediate redshifts (z=3). At higher redshifts (z>5) the model tendsto overestimate the data at faint luminosities. Critical biases both in thedata and in the model are discussed to explain such apparent discrepancies. Thepredicted hydrogen photoionization rate as a function of redshift is found tobe consistent with that derived from the observations. All that suggests toreconsider the role of the AGNs as the main driver of the ionization history ofthe Universe.

AGNs as main contributors to the UV ionizing emissivity at high redshifts: predictions from a Lambda-CDM model with linked AGN/galaxy evolution