The high-redshift He II Gunn-Peterson effect: Implications and future prospects

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 and z = 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 toward 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) x 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 hydrodynamic simulations of the Ly alpha absorber distribution. The He II observations are consistent with line blanketing from clouds having N(HI) greater-than-or-equal-to 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(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.