Radiative Transfer in a Clumpy Universe. II. The Ultraviolet Extragalactic Background. ApJ461

The integrated ultraviolet ux arising fromQSOs and/or hot, massive stars in metal-producing young galaxies is likely responsible for maintaining the intergalactic diuse gas and the Ly forest clouds in a highly ionized state. The spectrum and intensity of such UV background have generally been obtained by modeling the reprocessing due to intervening material as a pure photoelectric absorption process. However, owing to the emission from radiative recombinations within the ab-sorbing clouds, a photoionized clumpy medium could contribute substantially to the metagalactic ux. In other words, QSO absorption-line systems are sources not just sinks of ionizing photons. We present a detailed calculation of the propagation of AGN-like ionizing radiation through the intergalactic space. We model the ionization state of absorbing clouds, and show that the universe will be more opaque above 4Ryd than previously estimated. Singly ionized helium in Ly forest clouds and Lyman-limit systems is found to be very ecient in reprocessing soft X-ray, helium-ionizing photons into ultraviolet, hydrogen-ionizing ones. We demonstrate that a signicant fraction of the absorbed primary photons (emitted, e.g., by quasar sources) will be reradiated by the photoionized gas through Ly line emission, two-photon continuum, an