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The reionization epoch of singly ionized helium (He II) is believed to startat redshifts z~3.5--4 and be nearly complete by z~2.7. We explore thepost-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 asthey probe the post-reionization history of He II and the local ionizationenvironment around the quasar and transverse to the line of sight. We compareprevious spectra taken by the Far Ultraviolet Spectroscopic Explorer to thecurrent COS data, which have a substantially higher signal-to-noise ratio. TheGunn-Peterson trough recovers at lower redshifts, with the effective opticaldepth falling from {\tau}_eff~1.8 at z~2.7 to {\tau}_eff~0.7 at z~2.3. We seean interesting excess of flux near the He II Ly{\alpha} break, which could bequasar line emission, although likely not He II Ly{\alpha}. We present spectraof four possible transverse-proximity quasars, although the UV hardness dataare not of sufficient quality to say if their effects are seen along the HS1700sightline.

The He II Post-Reionization Epoch: HST/COS Observations of the Quasar HS1700+6416