Helium Reionization Simulations. III. The Helium Lyman-$\alpha$ Forest

TL;DR

This paper analyzes the observational signatures of helium II reionization in the Lyman-alpha forest, highlighting the flux PDF and power spectrum as key tools for understanding the reionization timeline and ionization state.

Contribution

It demonstrates that the flux PDF and flux power spectrum are effective in constraining helium II reionization timing and ionization state, beyond traditional optical depth measures.

Findings

Flux PDF can determine reionization timing.

Flux power spectrum indicates ionization state.

Optical depth alone is insufficient due to sightline variance.

Abstract

In La Plante et al. (2017), 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 $\tau_\mathrm{eff}$ is not sufficient for capturing the ionization state of helium II, due to the large variance inherent in sightlines. However, the He II flux 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 ($\sim$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.