INTERGALACTIC IONIZING FLUX FROM ACTIVE GALACTIC NUCLEI

Recently the appreciation that the X-ray background is the superposition of the X-ray emission from low-luminosity active galactic nuclei (AGNs), has been generalized. AGNs are also powerful emitters of the ultraviolet radiation and the intergalactic ionizing flux is possibly supplied by them. We evaluate the metagalactic ionizing flux from AGNs on the basis of the X-ray luminosity function and the luminosity evolution of AGNs. The predicted UV flux is consistent with the proximity effect on the Lyman-alpha clouds and the optical depth of intergalactic matter against the ionizing photons satisfy the Gunn-Peterson test for the high-redshift quasars. The evolution of the ionizing flux is also consistent with the evolution law determined by a pressure confinement model of the Lyman-alpha clouds.