Journal
ASTROPHYSICAL JOURNAL
Volume 692, Issue 2, Pages 1476-1488Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/692/2/1476
Keywords
cosmology: observations; diffuse radiation; galaxies: active; intergalactic medium
Categories
Funding
- NSF [AST-0407374, AST-0709356, AST-0239425, AST-0708793]
- University of Wisconsin Research Committee
- Wisconsin Alumni Research Foundation
- David and Lucile Packard Foundation
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We use a wide-field (0.9 deg(2)) X-ray sample with optical and Galaxy Evolution Explorer (GALEX) ultraviolet observations to measure the contribution of active galactic nuclei (Lambda GNs) to the ionizing flux as a function of redshift. Our analysis shows that the AGN contribution to the metagalactic ionizing background peaks at around z = 2. The measured values of the ionizing background from the AGNs are lower than previous estimates and confirm that ionization from AGNs is insufficient to maintain the observed ionization of the intergalactic medium (IGM) at z > 3. We show that only X-ray sources with broad lines in their optical spectra have detectable ionizing flux and that the ionizing flux seen in an AGN is not correlated with its Xray color. We also use the GALEX observations of the GOODS-N region to place a 2 sigma upper limit of 0.008 on the average ionization fraction f(nu)(700 (Lambda) over circle)/f(nu)(1500 (Lambda) over circle) for 626 UV selected galaxies in the redshift range z = 0.9-1.4. We then use this limit to estimate an upper bound to the galaxy contribution in the redshift range z = 0-5. If the z similar to 1.15 ionization fraction is appropriate for higher-redshift galaxies, then contributions from the galaxy population are also too low to account for the IGM ionization at the highest redshifts (z > 4).
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