HUBBLE SPACE TELESCOPE ∗ SPECTRAL OBSERVATIONS NEAR THE Heii Lyα BREAK: IMPLICATIONS FOR Heii REIONIZATION

Quasars that allow the study of intergalactic medium (IGM) Heii are very rare, since they must be at high redshift along sightlines free of substantial hydrogen absorption, but recent work has dramatically expanded the number of such quasars known. We analyze two dozen higher-redshift (z = 3.1–3.9) low-resolution Heii quasar spectra from the Hubble Space Telescope and find that their Heii Gunn–Peterson troughs suggest exclusion of very early and very late reionization models, favoring a reionization redshift of z ∼ 3. Although the data quality is not sufficient to reveal details such as the expected redshift evolution of helium opacity, we obtain the first ensemble measure of helium opacity at high redshift averaged over many sightlines: τ = 4.90 at z ∼ 3.3. We also find that it would be very difficult to observe the IGM red wing of absorption from the beginning of Heii reionization, but depending on the redshift of reionization and the size of ionization zones, it might be possible to do so in some objects with the current generation of UV spectrographs.