New Evidence for Extended He ii Reionization at z ≳ 3.5 from He ii Lyman Alpha and Beta Transmission Spikes*

Abstract

Abstract We present new high-resolution (R = λ/Δλ ∼ 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 ≳ 50 km s−1) transmission spikes in He ii Lyα and He ii Lyβ. The incidence of such spikes decreases with increasing redshift, but both sight lines show significant spikes at z ≳ 3.5, signaling the presence of fully ionized regions in the z ≳ 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 ΓHe II . From the incidence of Lyα and Lyβ transmission spikes we infer similar photoionization rates of at 3.51 < z < 3.66 and ΓHe II = (0.9 ± 0.3) × 10−15 s−1 at 3.460 < z < 3.685, respectively. Because the transmission spikes indicate fully ionized regions at z ≳ 3.5 along both lines of sight, our observations provide further evidence that He ii reionization had substantially progressed at these redshifts.