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

Abstract

The cosmological process of hydrogen (HI) 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 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 COSMOS data, capable to efficiently measure the 1 ryd comoving AGN emissivity up to $z\sim 5-6$ and down to five magnitudes fainter than probed by current optical surveys, without any luminosity extrapolation. We find good agreement between the logN${}_{\mathrm{H}}\lesssim 21-22$ cm${}^{-2}$ X-ray LF and the optically-selected QSO LF at all redshifts for $M_{1450}\le -23$. The full range of the logN${}_{\mathrm{H}}\lesssim 21-22$ cm${}^{-2}$ LF ($M_{1450}\le -17$) was then used to quantify the contribution of AGN to the photon budget critical value needed to keep the Universe ionized. We find that the contribution of ionizing AGN at $z=6$ is as small as 1\% - 7\%, and very unlikely to be greater than 30\%, thus excluding an AGN-dominated reionization scenario.

arXiv comment12 pages, 4 figures, 1 table, submitted to MNRAS after first referee revision

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