Semi-numeric simulations of helium reionization and the fluctuating radiation background

TL;DR

This paper introduces a fast semi-numeric simulation method to study helium reionization and the fluctuating radiation background, revealing large fluctuations in the He II ionization rate during and after reionization.

Contribution

It presents a new semi-numeric approach that efficiently models helium reionization, capturing fluctuations and bimodal distributions not seen in previous models.

Findings

Order-of-magnitude fluctuations in He II ionization rate post-reionization

Stronger, bimodal fluctuations during reionization

Low-level ionizing background persists at high He II fractions

Abstract

Recent He II Lyman-alpha forest observations from 2.0 < z < 3.2 show large fluctuations in the optical depth at z > 2.7. These results point to a fluctuating He-ionizing background, which may be due to the end of helium reionization of this era. We present a fast, semi-numeric procedure to approximate detailed cosmological simulations. We compute the distribution of dark matter halos, ionization state of helium, and density field at z = 3 in broad agreement with recent simulations. Given our speed and flexibility, we investigate a range of ionizing source and active quasar prescriptions. Spanning a large area of parameter space, we find order-of-magnitude fluctuations in the He II ionization rate in the post-reionization regime. During reionization, the fluctuations are even stronger and develop a bimodal distribution, in contrast to semi-analytic models and the hydrogen equivalent. These distributions indicate a low-level ionizing background even at significant He II fractions.