Measuring the Sources of the Intergalactic Ionizing Flux
  We use a wide-field (0.9 square degree) X-ray sample with optical and GALEX ultraviolet observations to measure the contribution of Active Galactic Nuclei (AGNs) to the ionizing flux as a function of redshift. Our analysis shows that the AGN contribution to the metagalactic ionizing background peaks 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 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 X-ray 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 fnu(700 A)/fnu(1500 A) 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~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). 
L. L. Cowie, A. J. Barger, L. Trouille