Abstract
We study the reionization of Helium II by quasars using a numerical approach that combines 3D radiative transfer calculations with cosmological hydrodynamical simulations. Sources producing the ionizing radiation are selected according to an empirical quasar luminosity function and are assigned luminosities according to their intrinsic masses. We present models in which these parameters are varied and examine characteristics of the resultant reionization process that distinguish the various cases. In addition, we extract artificial spectra from the simulations and quantify statistical properties of the spectral features in each model. We find that the most important factor affecting the evolution of He II reionization is the cumulative number of ionizing photons that are produced by the sources. Comparisons between He II opacities measured observationally and those obtained by our analysis reveal that the available ranges in plausible values for the parameters provide enough leeway to provide a satisfactory match. However, one property common to all our calculations is that the epoch of Helium II reionization must have occurred at a redshift between 3 < z < 4. If so, future observational programs will be able to directly trace the details of the ionization history of helium and probe the low density phase of the intergalactic medium during this phase of the evolution of the Universe.
Highlights
Space-based ultraviolet telescopes have made it possible to observe the Lyα transition of singly ionized helium (He II) along lines of sight to high-redshift quasars
We find that the most important factor affecting the evolution of He II reionization is the cumulative number of ionizing photons that are produced by the sources
In selecting the sources and computing their intensities we have introduced five free parameters associated with source characteristics, they are: (1) a universal source lifetime, Tlife, (2) a minimum mass, Mmin, (3) a minimum luminosity at z = 0, Lmin,o, (4) an angle specifying the beaming of the bi-polar radiation, β, and (5) a tail-end spectral index, αs, in the regime λ < 1050 ̊A
Summary
Space-based ultraviolet telescopes have made it possible to observe the Lyα transition of singly ionized helium (He II) along lines of sight to high-redshift quasars. The discovery of the “He II Gunn-Peterson effect” in Q0302-003 (z = 3.285) by Jakobsen et al (1994) marked the beginning of He II Lyα studies He II absorption in a second quasar, PKS 1935-692 (z = 3.18), was identified by Tytler (1995; see Jakobsen 1996), while Davidsen et al (1996) measured the He II opacity at a lower redshift of z = 2.72 towards the quasar HS 1700+6416 using the Hopkins Ultraviolet Telescope (HUT). Heap et al (2000) and Smette et al (2000) reported new HST/STIS spectra for Q0302-003 and HE 2347-4342, respectively These observations had sufficient resolution to begin to resolve some of the features in the He II Lyα forest and allowed them to to cross-correlate the data with the absorbers in the gaps of the hydrogen (H I) forest lines. Models of the spectra based on corresponding H I Lyα forests were presented to probe the hardness of the UV background, believed to emanate from the observed quasar population (e.g. Haardt & Madau 1996)
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