2021ApJ...912...38M

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 Γ_He II_. From the incidence of Lyα and Lyβ transmission spikes we infer similar photoionization rates of Γ_HeII=(2.0_–0.5^+0.6)×10^–15s^–1 at 3.51 < z < 3.66 and Γ_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 >= 3.5 along both lines of sight, our observations provide further evidence that He II reionization had substantially progressed at these redshifts.