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He II Lyβ Gunn-Peterson Absorption: New HST Observations, and Theoretical Expectations

Observations of He II Lyα Gunn-Peterson troughs have proved to be a valuable probe of the epoch of helium reionization at z~3. Since this optical depth can become unmeasurably large even for modest He II fractions, various alternate techniques have been proposed to push to higher redshift, and among the more promising is looking at higher order Lyman-series troughs. We here report four new observations of the He II Lyβ trough, including new data on the only sightline with a prior Lyβ observation. However, the effective optical depth ratio τ_eff,β/τ_eff,α is not simply predicted by (f_β λ_β)/(f_α λ_α)=0.16, and we analyze cosmological simulations to find that the correct ratio for helium at z~3 is ~0.35. In one case we infer τ_eff,α > 8.8, strong evidence that helium was not fully reionized at z=3.2--3.5, in agreement with previous measurements suggesting a later completion of reionization.