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

Abstract

The integrated ultraviolet ux arising from QSOs and/or hot, massive stars in metal-producing young galaxies is likely responsible for maintaining the intergalactic di use 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 absorbing 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 4 Ryd than previously estimated. Singly ionized helium in Ly forest clouds and Lyman-limit systems is found to be very e cient in reprocessing soft X-ray, helium-ionizing photons into ultraviolet, hydrogen-ionizing ones. We demonstrate that a signi cant 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, and recombination continuum radiation.In the light of new data and recent studies, we also reassess the contribution of the QSOs observed in optical surveys to the UV extragalactic background, and nd that the stochastic reprocessing of quasar Lyman continuum radiation by hydrogen and helium along the line of sight will signi cantly a ect the amplitude, spectral shape, and uctuations properties of the metagalactic ux. In a scenario in which QSOs are the primary source of ionizing photons in the universe, the integrated H I Ly emission at z = 0 from photoionized Ly clouds and Lymanlimit systems is found to be at a level of less than 5% of current observational limits on the far-UV extragalactic radiation ux. We show that J 912 increases from 10 23 ergs cm 2 s 1 Hz 1 sr 1 at the present epoch to 5 10 22 ergs cm 2 s 1 Hz 1 sr 1 at z = 2:5. The attenuated direct quasar emission plus recombination radiation from intergalactic gas appears to provide enough hydrogen-ionizing photons to satisfy the proximity e ect at large redshift. The He II /H I ratio in the di use intergalactic medium and the Ly clouds increases from 25 at z = 0 to 45 at z = 2:5, to decrease again below 30 for z > 4:5. The spectrum of the ionizing background at high redshift is shown to have a hump at energies below 40.8 eV due to redshift-smeared He II Ly line and two-photon continuum emission. We propose that observations of low-ionization species such { 3 {

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