Constraints on the UV metagalactic emissivity using the LyA forest

Abstract

Numerical hydrodynamical simulations have proven a successful means of reproducing many of the statistical properties of the LyA forest as measured in high redshift (z > 4) quasar spectra. The source of ionization of the Intergalactic Medium (IGM), however, remains unknown. We investigate how the LyA forest may be used to probe the nature of the sources. We show that the attenuation of Lyman continuum photons by the IGM depends sensitively on the emissivity of the sources, permitting a strong constraint to be set on the required emissivity to match the measured values of the mean IGM LyA optical depth. We also investigate the effect of fluctuations in the UV background. We derive the distribution function of the background radiation field produced by discrete sources in an infinite universe, including the effects of attenuation by an intervening absorbing medium. We show that for z > 5, the fluctuations boost the mean LyA optical depth in a scenario in which QSO sources dominate the UV background. The fluctuations will also result in large spatial correlations in the ionization level of the IGM at high redshifts, providing a means of discriminating between scenarios in which stars and galaxies dominate the UV background vs a QSO-dominated background.Comment: 11 pages, 8 figures. Accepted for publication in MNRA