The HeI 584 A Forest as a Diagnostic of Helium Reionization

TL;DR

This paper explores using the HeI 584 A forest as a novel method to detect and analyze helium reionization in the universe, proposing observational strategies with current telescopes.

Contribution

It introduces a new diagnostic technique using HeI 584 A absorption to study HeII reionization and outlines observational requirements for detection.

Findings

HeI 584 A absorption can indicate HeII reionization.

Detection requires spectral resolution of 10^4 and S/N ratio of ~10.

Potential for observations with Hubble's Cosmic Origins Spectrograph.

Abstract

We discuss the potential of using the HeI 584 A forest to detect and study HeII reionization. Significant 584 A absorption is expected from intergalactic HeII regions, whereas there should be no detectable absorption from low density gas in HeIII regions. Unlike HeII Ly-alpha absorption (the subject of much recent study), the difficulty with using this transition to study HeII reionization is not saturation but rather that the absorption is weak. The Gunn-Peterson optical depth for this transition is tau ~ 0.1 x_{HeII} Delta^2 [(1+z)/5]^{9/2}, where x_{HeII} is the fraction of helium in HeII and Delta is the density in units of the cosmic mean. In addition, HeI 584 A absorption is contaminated by lower redshift HI Ly-alpha absorption with a comparable flux decrement. We estimate the requirements for a definitive detection of redshifted HeI absorption from low density gas (Delta ~ 1), which would indicate that HeII reionization was occurring. We find that this objective can be accomplished (using coeval HI Ly-alpha absorption to mask dense regions and in cross correlation) with a spectral resolution of 10^4 and a signal-to-noise ratio per resolution element of ~ 10. Such specifications may be achievable on a few known z ~ 3.5 quasar sightlines with the Cosmic Origins Spectrograph on the Hubble Space Telescope. We also discuss how HeI absorption can be used to measure the hardness of the ionizing background above 13.6 eV.