UCLA

Recent He II Lyman a forest observations within the range 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 haloes, ionization state of helium, and density field at z = 3, which are 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. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.