2006MNRAS.366.1378B

The low-density hydrogen and helium in the intergalactic medium (IGM) probed by quasi-stellar object (QSO) absorption lines is sensitive to the amplitude and spectral shape of the metagalactic ultraviolet (UV) background. We use realistic HI and HeII Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of a Λ cold dark matter (ΛCDM) universe to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic HI and HeII ionization rates. We further show that the large spatial variations and the anticorrelation with HI absorber density observed in the ratio of the measured Heii to HI column densities can be explained in a model where the HI ionization rate is dominated by the combined UV emission from young star-forming galaxies and QSOs and the Heii ionization rate is dominated by emission from QSOs only. In such a model the large fluctuations in the column density ratio are due to the small number of QSOs expected to contribute at any given point to the Heii ionization rate. A significant contribution to UV emission at the Heii photoelectric edge from hot gas in galaxies and galaxy groups would decrease the expected fluctuations in the column density ratio. Consequently, this model appears difficult to reconcile with the large increase in Heii opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed Heii Lyα forest spectra at z∼ 2-3.5 probe the tail end of the reionization of Heii by QSOs.