  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 HI and HeII Ly-alpha forest spectra, constructed
from state-of-the-art hydrodynamical simulations of a Lambda-CDM 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 anti-correlation 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 Ly-alpha forest spectra at z~2-3.5 probe the tail end of the reionization
of HeII by QSOs.
