%0 Generic %1 haardt2015redshift %A Haardt, Francesco %A Salvaterra, Ruben %D 2015 %K AGN reionization %T High redshift AGNs and HI reionisation: limits from the unresolved X-ray background %U http://arxiv.org/abs/1502.03089 %X The rapidly declining population of bright quasars at z~3 appears to make an increasingly small contribution to the ionising background at the HI Lyman limit. It is then generally though that massive stars in (pre-)galactic systems may provide the additional ionising flux needed to complete HI reionisation by z>6. A galaxy dominated background, however, may require that the escape fraction of Lyman continuum radiation from high redshift galaxies is as high as 10%, a value somewhat at odds with (admittedly scarce) observational constraints. High escape fractions from dwarf galaxies have been advocated, or, alternatively, a so-far undetected (or barely detected) population of unobscured, high-redshift faint AGNs. Here we question the latter hypothesis, and show that such sources, to be consistent with the measured level of the unresolved X-ray background at z=0, can provide a fraction of the HII filling factor not larger than 13% by z=6. The fraction rises to <27% in the somewhat extreme case of a constant comoving redshift evolution of the AGN emissivity. This still calls for a mean escape fraction of ionising photons from high-z galaxies >10%.

@misc{haardt2015redshift, abstract = {The rapidly declining population of bright quasars at z~3 appears to make an increasingly small contribution to the ionising background at the HI Lyman limit. It is then generally though that massive stars in (pre-)galactic systems may provide the additional ionising flux needed to complete HI reionisation by z>6. A galaxy dominated background, however, may require that the escape fraction of Lyman continuum radiation from high redshift galaxies is as high as 10%, a value somewhat at odds with (admittedly scarce) observational constraints. High escape fractions from dwarf galaxies have been advocated, or, alternatively, a so-far undetected (or barely detected) population of unobscured, high-redshift faint AGNs. Here we question the latter hypothesis, and show that such sources, to be consistent with the measured level of the unresolved X-ray background at z=0, can provide a fraction of the HII filling factor not larger than 13% by z=6. The fraction rises to <27% in the somewhat extreme case of a constant comoving redshift evolution of the AGN emissivity. This still calls for a mean escape fraction of ionising photons from high-z galaxies >10%.}, added-at = {2015-02-11T09:45:23.000+0100}, author = {Haardt, Francesco and Salvaterra, Ruben}, biburl = {https://www.bibsonomy.org/bibtex/25d53946d19523f9a0474bbf3ce623336/miki}, description = {[1502.03089] High redshift AGNs and HI reionisation: limits from the unresolved X-ray background}, interhash = {7b220ca1ec338d622f60b2ceae7177f5}, intrahash = {5d53946d19523f9a0474bbf3ce623336}, keywords = {AGN reionization}, note = {cite arxiv:1502.03089Comment: A&A Letter, accepted (4 pages, 2 figures)}, timestamp = {2015-02-11T09:45:23.000+0100}, title = {High redshift AGNs and HI reionisation: limits from the unresolved X-ray background}, url = {http://arxiv.org/abs/1502.03089}, year = 2015 }