Helium Reionization Simulations. III. The Helium Lyα Forest

In a previous paper we presented a new suite of hydrodynamic simulations with the aim of accurately capturing the process of helium II reionization. In this paper, we discuss the observational signatures present in the He II Ly alpha forest. We show that the effective optical depth of the volume-averaged tau(eff) alone is not sufficient for capturing the ionization state of helium II, due to the large variance inherent in sightlines. However, the He II flux probability density function (PDF) can be used to determine the timing of helium II reionization. The amplitude of the one-dimensional flux power spectrum can also determine the ionization state of helium II. We show that, even given the currently limited number of observations (similar to 50 sightlines), measurements of the flux PDF can yield information about helium II reionization. Further, measurements using the one-dimensional power spectrum can provide clear indications of the timing of reionization, as well as the relative bias of sources of ionizing radiation.