Constraining the UV emissivity of AGN throughout cosmic time via X-ray surveys

The cosmological process of hydrogen (H I) reionization in the intergalactic medium is thought to be driven by UV photons emitted by star-forming galaxies and ionizing active galactic nuclei (AGN). The contribution of quasars (QSOs) to HI reionization at z > 4 has been traditionally believed to be quite modest. However, this view has been recently challenged by new estimates of a higher faint-end UV luminosity function (LF). To set firmer constraints on the emissivity of AGN at z < 6, we here make use of complete X-ray-selected samples including deep Chandra and new Cosmic Evolution Survey data, capable to efficiently measure the 1 Ryd comoving AGN emissivity up to z similar to 5-6 and down to 5 mag fainter than probed by current optical surveys, without any luminosity extrapolation. We find good agreement between the logN(H) less than or similar to 21-22 cm(-2) X-ray LF and the optically selected QSO LF at all redshifts for M-1450 <= -23. The full range of the logN(H) less than or similar to 21-22 cm(-2) LF (M-1450 <= -17) was then used to quantify the contribution of AGN to the critical value of photon budget needed to keep the Universe ionized. We find that the contribution of ionizing AGN at z = 6 is as small as 1-7 per cent, and very unlikely to be greater than 30 per cent, thus excluding an AGN-dominated reionization scenario.