Absorption due to He II Ly \alpha has now been detected at low resolution in the spectra of four quasars between redshifts z = 2.74 – 3.29 .
We assess these observations , giving particular attention to the radiative transfer of the ionizing background radiation , cloud diffuse emission and ionization physics , and statistical fluctuations .
We use high-resolution observations of H I absorption towards quasars to derive an improved model for the opacity of the intergalactic medium ( IGM ) from the distribution of absorbing clouds in column density and redshift .
We use these models to calculate the H I and He II photoionization history , the ratio \eta = He II/H I in both optically-thin and self-shielded clouds , and the average line-blanketing contribution of the clouds to He II absorption .
The derived ionization rate , \Gamma _ { HI } = ( 1 - 3 ) \times 10 ^ { -12 } s ^ { -1 } ( z = 2 - 4 ) is consistent with the ionizing background intensity inferred from the “ proximity effect ” , but it remains larger than that inferred by N-body hydrodynamical simulations of the Ly \alpha absorber distribution .
The He II observations are consistent with line blanketing from clouds having N _ { \it HI } \geq 10 ^ { 12 } cm ^ { -2 } , although a contribution from a more diffuse IGM would help to explain the observed opacity .
We compute the expected He II optical depth , \tau _ { \it HeII } ( z ) , and examine the implications of the sizable fluctuations that arise from variations in the cloud numbers and ionizing radiation field .
We assess how He II absorption constrains the intensity and spectrum of the ionizing radiation and the fractional contributions of the dominant sources ( quasars and starburst galaxies ) .
Finally , we demonstrate how high-resolution ultraviolet observations can distinguish between absorption from the diffuse IGM and the Ly \alpha forest clouds and determine the source of the ionizing background .