Spatial fluctuations in the spectral shape of the UV background at 2 < z < 3 and the reionization of helium

The low density hydrogen and helium in the IGM probed by QSO absorption lines is sensitive to the amplitude and spectral shape of the metagalactic UV background. We use realistic H I and He II Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of a ΛCDM Universe, to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic H I and He II ionization rates. We further show that the large spatial variations and the anti-correlation with H I absorber density observed in the ratio of the measured He II to H I column densities can be explained in a model where the H I ionization rate is dominated by the combined UV emission from young star forming galaxies and QSOs and the He II 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 He II ionization rate. A significant contribution to UV emission at the He II 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 He II opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed He II Lyα forest spectra at z ∼ 2− 3.5 probe the tail end of the reionization of He II by QSOs.