2009ApJ...704L..89M

Many experts believe that the z ∼ 3 He II Lyα forest will suffer from the same saturation issues as the z ∼ 6 H I Lyα forest and, therefore, will not be a sensitive probe of He II reionization. However, there are several factors that make He II Lyα absorption more sensitive than H I Lyα. We show that observations of He II Lyα and Lyβ Gunn-Peterson troughs can provide a relatively model-independent constraint on the volume-averaged He II fraction of. This bound derives from first using the most underdense regions in the He II forest to constrain the local He II fraction and, then, assuming photoionization equilibrium with the maximum allowed photoionization rate to calculate the ionization state of nearby gas. It is possible to evade this constraint by a factor of ∼2, but only if the He II were reionized recently. We argue that He II Lyα Gunn-Peterson troughs observed in the spectra of Q0302-003 and HE2347-4342 signify the presence of ≳10 comoving Mpc patches in which. This is a factor of 20 improvement over previous constraints from these spectra and 100 times stronger than the tightest constraint on the H I volume-filling fraction from the z > 6 H I Lyman forest.