HeII Ly$\alpha$ Transmission Spikes and Absorption Troughs in Eight High-resolution Spectra Probing the End of HeII Reionization

TL;DR

This study analyzes high-resolution spectra of quasars to investigate the end of HeII reionization, revealing evolving absorption features and constraining the HeII photoionization rate during this epoch.

Contribution

It provides new measurements of HeII absorption troughs and transmission spikes, and infers the redshift evolution of the HeII photoionization rate using hydrodynamical simulations.

Findings

Trough lengths decrease from >100 cMpc at z>3 to ~30 cMpc at z~2.7

Detection of long troughs (~65 cMpc) at z~2.9 suggests large-scale UV background fluctuations

HeII photoionization rate declines from 4.6×10^{-15} s^{-1} at z~2.6 to 1.2×10^{-15} s^{-1} at z~3.2

Abstract

We present statistics of HeII Lya transmission spikes and large-scale absorption troughs using archival high-resolution ($R=\lambda /\Delta \lambda \simeq 12,500$-$18,000$) far-UV spectra of eight HeII-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 HeII absorption at the end of the HeII reionization epoch. The measured lengths of the troughs decrease dramatically from L>100cMpc at z>3 to L~30cMpc at z~2.7, signaling a significant progression of HeII reionization at these redshifts. Furthermore, unexpectedly long L~65cMpc 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 (146cMpc)^3 optically thin Nyx hydrodynamical simulation employing different UV background models, we infer the redshift evolution of the HeII photoionization rate $\Gamma_\mathrm{He\,II}(z)$. The photoionization rate decreases with increasing redshift from $\simeq 4.6\times 10^{-15}\mathrm{\,s^{-1}}$ at z~2.6 to $\simeq 1.2 \times 10^{-15}\mathrm{\,s^{-1}}$ at z~3.2, in agreement with previous inferences from the HeII effective optical depth, and following expected trends of current models of a fluctuating HeII-ionizing background.