He ii Lyα Transmission Spikes and Absorption Troughs in Eight High-resolution Spectra Probing the End of He ii Reionization

Abstract

Abstract 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 ΓHe II (z). The photoionization rate decreases with increasing redshift from ≃ 4.6 × 10−15 s−1 at z ≃ 2.6 to ≃ 1.2 × 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.