Please use this identifier to cite or link to this item:
|Title:||High-redshift active galactic nuclei and H I reionisation: limits from the unresolved X-ray background||Authors:||HAARDT, FRANCESCO
|Issue Date:||2015||Journal:||ASTRONOMY & ASTROPHYSICS||Number:||575||First Page:||L16||Abstract:||The rapidly declining population of bright quasars at z ≳ 3 appears to make an increasingly smaller contribution to the ionising background at the H I Lyman limit. It is thererfore generally thought that massive stars in (pre-)Galactic systems may provide the additional ionising flux needed to complete H I 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%, which is 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 examine 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 H II filling factor no 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 of ≳10%. <P />||Acknowledgments:||We thank A. Comastri, P. Madau, A. Moretti for many fruitful discussions, and E. Giallongo for allowing us to use their results before publication.||URI:||http://hdl.handle.net/20.500.12386/23397||URL:||https://www.aanda.org/articles/aa/abs/2015/03/aa25627-15/aa25627-15.html||ISSN:||0004-6361||DOI:||10.1051/0004-6361/201525627||Bibcode ADS:||2015A&A...575L..16H||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
Show full item record
Items in DSpace are published in Open Access, unless otherwise indicated.