%0 Generic %1 parsa2017evidence %A Parsa, S. %A Dunlop, J. S. %A McLure, R. J. %D 2017 %K AGN high-redshift reionization %T No evidence for a significant AGN contribution to cosmic hydrogen reionization %U http://arxiv.org/abs/1704.07750 %X We reinvestigate a claimed sample of 22 X-ray detected active galactic nuclei (AGN) at redshifts z > 4, which has reignited the debate as to whether young galaxies or AGN reionized the Universe. These sources lie within the GOODS-S/CANDELS field, and we examine both the robustness of the claimed X-ray detections (within the Chandra 4Ms imaging) and perform an independent analysis of the photometric redshifts of the optical/infrared counterparts. We confirm the reality of only 15 of the 22 reported X-ray detections, and moreover find that only 12 of the 22 optical/infrared counterpart galaxies actually lie robustly at z > 4. Combining these results we find convincing evidence for only 7 X-ray AGN at z > 4 in the GOODS-S field, of which only one lies at z > 5. We recalculate the evolving far-UV (1500 Angstrom) luminosity density produced by AGN at high redshift, and find that it declines rapidly from z = 4 to z = 6, in agreement with several other recent studies of the evolving AGN luminosity function. The associated rapid decline in inferred hydrogen-ionizing emissivity contributed by AGN falls an order-of-magnitude short of the level required to maintain hydrogen ionization at z ~ 6. We conclude that all available evidence continues to favour a scenario in which young galaxies reionized the Universe, with AGN making, at most, a very minor contribution to cosmic hydrogen reionization.

@misc{parsa2017evidence, abstract = {We reinvestigate a claimed sample of 22 X-ray detected active galactic nuclei (AGN) at redshifts z > 4, which has reignited the debate as to whether young galaxies or AGN reionized the Universe. These sources lie within the GOODS-S/CANDELS field, and we examine both the robustness of the claimed X-ray detections (within the Chandra 4Ms imaging) and perform an independent analysis of the photometric redshifts of the optical/infrared counterparts. We confirm the reality of only 15 of the 22 reported X-ray detections, and moreover find that only 12 of the 22 optical/infrared counterpart galaxies actually lie robustly at z > 4. Combining these results we find convincing evidence for only 7 X-ray AGN at z > 4 in the GOODS-S field, of which only one lies at z > 5. We recalculate the evolving far-UV (1500 Angstrom) luminosity density produced by AGN at high redshift, and find that it declines rapidly from z = 4 to z = 6, in agreement with several other recent studies of the evolving AGN luminosity function. The associated rapid decline in inferred hydrogen-ionizing emissivity contributed by AGN falls an order-of-magnitude short of the level required to maintain hydrogen ionization at z ~ 6. We conclude that all available evidence continues to favour a scenario in which young galaxies reionized the Universe, with AGN making, at most, a very minor contribution to cosmic hydrogen reionization.}, added-at = {2017-04-26T10:03:20.000+0200}, author = {Parsa, S. and Dunlop, J. S. and McLure, R. J.}, biburl = {https://www.bibsonomy.org/bibtex/285f4753095fb4112e774f53f407828b3/miki}, description = {[1704.07750] No evidence for a significant AGN contribution to cosmic hydrogen reionization}, interhash = {60267de13b91ee8f34d51d3621463cde}, intrahash = {85f4753095fb4112e774f53f407828b3}, keywords = {AGN high-redshift reionization}, note = {cite arxiv:1704.07750Comment: 20 pages, 8 figures, submitted to MNRAS}, timestamp = {2017-04-26T10:03:20.000+0200}, title = {No evidence for a significant AGN contribution to cosmic hydrogen reionization}, url = {http://arxiv.org/abs/1704.07750}, year = 2017 }