HST/COS Observations of Thirteen New He II Quasars
  The full reionization of intergalactic helium was a major event in the
history of the IGM, and UV observations of the He II Gunn-Peterson trough allow
us to characterize the end of this process at z~3. Due to intervening hydrogen
absorption, quasars allowing such study are rare, with only 33 known in the
literature, and most of those are very recent discoveries. We expand on our
previous discovery work, and present 13 new He II quasars with redshifts 2.82 <
z < 3.77, here selected with ~80% efficiency, and including several that are
much brighter than the vast majority of those previously known. This is the
largest sample of uniformly observed He II quasars covering such a broad
redshift range, and they show evidence of IGM opacity increasing with redshift,
as expected for the helium reionization epoch. No evidence of He II Ly{\alpha}
quasar emission is seen in individual or averaged spectra, posing a problem for
standard models of the broad line region. The current rapid advance in the
study of He II quasars has been greatly facilitated by the Cosmic Origins
Spectrograph on HST, and we discuss the instrumental and other subtleties that
must be taken into account in IGM He II observations.
