THE He II POST-REIONIZATION EPOCH: HST/COS OBSERVATIONS OF THE QUASAR HS1700+6416

The reionization epoch of singly ionized helium (He II) is believed to start at redshifts z similar to 3.5-4 and be nearly complete by z similar or equal to 2.7. We explore the post-reionization epoch with far-ultraviolet spectra of the bright, high-redshift quasar HS1700+6416 taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope, which show strong He II (lambda 303.78) absorption shortward of the QSO redshift, z(QSO) = 2.75. We discuss these data as they probe the post-reionization history of He II and the local ionization environment around the quasar and transverse to the line of sight, finding that quasars are likely responsible for much of the ionization. We compare previous spectra taken by the Far-Ultraviolet Spectroscopic Explorer to the current COS data, which have a substantially higher signal-to-noise ratio. The Gunn-Peterson trough recovers at lower redshifts, with the effective optical depth falling from tau(eff) similar or equal to 1.8 at z similar to 2.7 to tau(eff) similar or equal to 0.7 at z similar to 2.3, higher than has been reported in earlier work. We see an interesting excess of flux near the He II Ly alpha break, which could be quasar line emission, although likely not He II Ly alpha. We present spectra of four possible transverse-proximity quasars, although the UV hardness data are not of sufficient quality to say if their effects are seen along the HS1700 sightline.