UCLA

Recent observations suggest that quasars fully ionize helium around redshift z ~ 3. Most directly, the optical depth derived from the He II Lyman-α (Lyα) forest fluctuates significantly above and near z ~ 2.8 and then evolves smoothly at lower redshifts. These fluctuations may be indicative of patchy He II reionization, which is expected (primarily) due to the rarity and brightness of quasars and the inhomogeneous intergalactic medium. This dissertation presents approximate methods for interpreting observations of this epoch and quantifying the magnitude of fluctuations.

From these opacity measurements, we compute, using a semi-analytic model, the evolution of the mean photoionization rate and attenuation length for helium over the redshift range 2.0 ≤ z ≤ 3.2. The average effective optical depth indicates a sharp decrease in the photoionization rate and, provided the quasar emissivity evolves smoothly, in the effective attenuation length at z ~ 2.8. A simple model with reionization ending at z ~ 2.7 is most consistent with the data, although the constraints are weak thanks to sparse, uncertain data.

We further present a fast, semi-numeric procedure to approximate detailed cosmological simulations. We produce 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 source and active quasar prescriptions. We find order-of-magnitude fluctuations in the He II ionization rate in the post-reionization Universe. During reionization, the fluctuations are even stronger and develop a bimodal distribution, in contrast with semi-analytic models and the hydrogen equivalent. We also examine large-scale fluctuations in the He II Lyα forest transmission during and after He II reionization. Like previous studies, we show that the rarity of quasars and the inhomogeneous intergalactic medium induce order unity fluctuations in the mean optical depth after reionization, even when averaged over large segments (~10-100 Mpc in length). During reionization, we find a broader distribution with an extended high-opacity tail. These conclusions are fairly insensitive underlying model assumptions, including quasar properties and the mean free path of photons. We compare to existing data along ~10 He II Lyα forest lines of sight spanning z ~ 2 - 3.2, demonstrating that the observed fluctuations are likely due to ongoing reionization.