2022ApJ...927..175M

We present statistics of He II Lyα transmission spikes and large-scale absorption troughs using archival high-resolution (R = λ/Δλ ≃ 12,500-18,000) far-UV spectra of eight He II-transparent quasars obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. The sample covers the redshift range 2.5 <= z <= 3.8, thereby probing the rapidly evolving He II absorption at the end of He II reionization epoch. The measured lengths of the troughs decrease dramatically from L >= 100 cMpc at z > 3 to L ≃ 30 cMpc at z ∼ 2.7, signaling a significant progression of He II reionization at these redshifts. Furthermore, unexpectedly long L ∼ 65 cMpc troughs detected at z ≃ 2.9 suggest that the UV background fluctuates at larger scales than predicted by current models. By comparing the measured incidence of transmission spikes to predictions from forward-modeled mock spectra created from the outputs of a (146 cMpc)³ optically thin Nyx hydrodynamical simulation employing different UV background models, we infer the redshift evolution of the He II photoionization rate Γ_He II_(z). The photoionization rate decreases with increasing redshift from ≃ 4.6 x 10^–15 s^–1 at z ≃ 2.6 to ≃ 1.2 x 10^–15 s^–1 at z ≃ 3.2, in agreement with previous inferences from the He II effective optical depth, and following expected trends of current models of a fluctuating He II-ionizing background.