The Fluctuating Intergalactic Ionizing Background Across Cosmic Time

Understanding the ionizing background of the universe is crucial to interpreting observations of intergalactic gas in the context of large-scale structure. The epochs of H and He reionization, currently under intense observational and theoretical investigation, set the boundary conditions for the propagation of ionizing photons in the universe. Using novel 1D and semi-numerical 3D calculations, we find that fluctuations in the ionizing background due to rare or clustered sources can be very important, in contrast to common assumptions in previous work. We show that fluctuations in the radiation field cause the mean free path of ionizing photons to vary, leading to large-scale correlations that may explain recent observations of the H and He Lyman-alpha forests following their respective reionization epochs.

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