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

Abstract

The integrated ultraviolet flux arising from QSOs and/or hot, massive stars in metal-producing young galaxies is likely responsible for maintaining the intergalactic diffuse gas and the Lyα 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 flux. 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 Ry than previously estimated. Singly ionized helium in Lyα forest clouds and Lyman-limit systems is found to be very efficient in reprocessing soft X-ray, helium-ionizing photons into ultraviolet, hydrogen-ionizing ones. We demonstrate that a significant fraction of the absorbed primary photons (emitted, e.g., by quasar sources) will be reradiated by the photoionized gas through Lyα 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 find that the stochastic reprocessing of quasar Lyman continuum radiation by hydrogen and helium along the line of sight will significantly affect the amplitude, spectral shape, and fluctuation properties of the metagalactic flux. In a scenario in which QSOs are the primary source of ionizing photons in the universe, the integrated H I Lyα emission at z = 0 from photoionized Lyα clouds and Lyman limit systems is found to be at a level of less than 5% of current observational limits on the far-UV extragalactic radiation flux. We show that J_912_ increases from ~10^-23^ ergs cm^-2^ s^-1^ Hz^- 1^ sr^-1^ at the present epoch to ~5 x 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 appear to provide enough hydrogen-ionizing photons to satisfy the proximity effect at large redshift. The He II/H I ratio in the diffuse intergalactic medium and the Lyα 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 Il Lyα line and two-photon continuum emission. We propose that observations of low-ionization species such as O II in metal-line absorption systems may be able to test the presence of such a prominent feature in the UV background spectrum. We also note that, if the metagalactic flux is dominated by QSOs, the suggested steep decline of their ionizing emissivity beyond z ~ 4 should produce an increase in the observed rate of incidence of Lyα forest clouds at these redshifts relative to an extrapolation from the intermediate-z regime, as observed by Williger et al. (1994).