2010MNRAS.408.1945M

We discuss the potential of using the Hei 584 Å forest to detect and study Heii reionization. Significant 584 Å absorption is expected from intergalactic Heii regions, whereas there should be no detectable absorption from low-density gas in Heiii regions. Unlike Heii Lyα 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 τ ∼ 0.1x_HeiiΔ²[(1 + z)/5]^9/2, where x_Heii is the fraction of helium in Heii and Δ is the density in units of the cosmic mean. In addition, Hei 584 Å absorption is contaminated by lower redshift HI Lyα absorption with a comparable flux decrement. We estimate the requirements for a definitive detection of redshifted Hei absorption from low-density gas (Δ ~ 1), which would indicate that Heii reionization was occurring. We find that this objective can be accomplished (using coeval HI Lyα 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 ∼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.