HST Spectral Observations near the He II Ly-alpha Break: Implications
  for He II Reionization
  Quasars that allow the study of IGM He II 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 He II quasar
spectra from HST and find that their He II 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: tau=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 He II
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.
