The He I 584 Å forest as a diagnostic of helium reionization

We discuss the potential of using the He i 584 A forest to detect and study He ii reionization. Significant 584 A absorption is expected from intergalactic He ii regions, whereas there should be no detectable absorption from low-density gas in He iii regions. Unlike He ii Ly alpha absorption (the subject of much recent study), the difficulty with using this transition to study He ii reionization is not saturation but rather that the absorption is weak. The Gunn-Peterson optical depth for this transition is tau similar to 0.1x(He ii) delta 2 [(1 + z)/5]9/2, where x(He ii) is the fraction of helium in He ii and delta is the density in units of the cosmic mean. In addition, He i 584 A absorption is contaminated by lower redshift H i Ly alpha absorption with a comparable flux decrement. We estimate the requirements for a definitive detection of redshifted He i absorption from low-density gas (delta approximate to 1), which would indicate that He ii reionization was occurring. We find that this objective can be accomplished (using coeval H i Ly alpha absorption to mask dense regions and in cross-correlation) with a spectral resolution of 104 and a signal-to-noise ratio per resolution element of similar to 10. Such specifications may be achievable on a few known z similar to 3.5 quasar sightlines with the Cosmic Origins Spectrograph on the Hubble Space Telescope. We also discuss how He i absorption can be used to measure the hardness of the ionizing background above 13.6 eV.