HST Spectral Observations near the He II Ly-alpha Break: Implications for He II Reionization

TL;DR

This study uses HST spectral data of high-redshift quasars to analyze helium reionization, constraining its timing around redshift 3 and providing the first ensemble measure of helium opacity at these epochs.

Contribution

It presents the first ensemble measurement of helium opacity at high redshift and constrains the timing of He II reionization using a large sample of quasar spectra.

Findings

Reionization likely occurred around redshift z~3.

Helium opacity at z~3.3 is approximately 4.90.

Early or late reionization models are inconsistent with the data.

Abstract

Quasars that allow the study of IGM He II are very rare, since they must be at high redshift along sightlines free of substantial hydrogen absorption, but recent work has dramatically expanded the number of such quasars known. We analyze two dozen higher-redshift (z=3.1-3.9) low-resolution He II quasar spectra from HST and find that their He II Gunn-Peterson troughs suggest exclusion of very early and very late reionization models, favoring a reionization redshift of z~3. Although the data quality is not sufficient to reveal details such as the expected redshift evolution of helium opacity, we obtain the first ensemble measure of helium opacity at high redshift averaged over many sightlines: tau=4.90 at z~3.3. We also find that it would be very difficult to observe the IGM red wing of absorption from the beginning of He II reionization, but depending on the redshift of reionization and the size of ionization zones, it might be possible to do so in some objects with the current generation of UV spectrographs.