He II Lyβ GUNN-PETERSON ABSORPTION: NEW HST OBSERVATIONS AND THEORETICAL EXPECTATIONS

Observations of He II Ly alpha Gunn-Peterson troughs have proved to be a valuable probe of the epoch of helium reionization at z similar to 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 Ly beta trough, including new data on the only sightline with a prior Ly beta observation. However, the effective optical depth ratio tau(eff,beta)/tau(eff,alpha) is not simply predicted by f(beta)lambda(beta)/f(alpha)lambda(alpha) = 0.16, and we analyze cosmological simulations to find that the correct ratio for helium at z similar to 3 is similar or equal to 0.35. In one case we infer tau(eff,alpha) > 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.