New Evidence For Extended He Ii Reionization At Z Greater Than Or Similar To 3.5 From He Ii Lyman Alpha And Beta Transmission Spikes

We present new high-resolution (R = lambda/Delta lambda similar to 14,000) spectra of the two brightest He ii-transparent quasars in the far-UV at z(em) > 3.5, HE2QS J2311-1417 (z(em) = 3.70) and HE2QS J1630+0435 (z(em) = 3.81), obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. In the predominantly saturated He ii absorption spectra, both sight lines show several isolated resolved (FWHM greater than or similar to 50 km s(-1)) transmission spikes in He ii Ly alpha and He ii Ly beta. The incidence of such spikes decreases with increasing redshift, but both sight lines show significant spikes at z greater than or similar to 3.5, signaling the presence of fully ionized regions in the z greater than or similar to 3.5 intergalactic medium (IGM). We employ an automated algorithm to compare the number of detected He ii transmission spikes to predictions from mock spectra created from the outputs of a cubic (146 cMpc)(3) optically thin Nyx hydrodynamical simulation, assuming a range of UV background photoionization rates Gamma(He II). From the incidence of Ly alpha and Ly beta transmission spikes we infer similar photoionization rates of Gamma(He II) = -(2.0(-0.5)(+0.6)) x 10(-15) s(-1) at 3.51 < z < 3.66 and Gamma(He II) = (0.9 +/- 0.3) x 10(-15) s(-1) at 3.460 < z < 3.685, respectively. Because the transmission spikes indicate fully ionized regions at z greater than or similar to 3.5 along both lines of sight, our observations provide further evidence that He ii reionization had substantially progressed at these redshifts.