HST/COS observations of thirteen new HeII quasars

The full reionization of intergalactic helium was a major event in the history of the intergalactic medium (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 IIquasars 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 IIquasars 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 Lyα 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 the Hubble Space Telescope, and we discuss the instrumental and other subtleties that must be taken into account in IGM He II observations.