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Abstract

Recent observations have shown that the scatter in opacities amongst coeval segments of the Ly α forest increases rapidly at z > 5. In this paper, we assess whether the large scatter can be explained by fluctuations in the ionizing background in the post-reionization intergalactic medium. We find that matching the observed scatter at z ≈ 5.5 requires a short spatially averaged mean free path of 〈λ_mfp⁹¹²〉 ≲ 15 h^{- 1} comoving Mpc, a factor of ≳3 shorter than direct measurements at z = 5.2. We argue that such rapid evolution in the mean free path is difficult to reconcile with our measurements of the global H I photoionization rate, which stay approximately constant over the interval z ≈ 4.8-5.5. However, we also show that measurements of the mean free path at z > 5 are likely biased towards higher values by the quasar proximity effect. This bias can reconcile the short values of 〈λ_mfp⁹¹²〉 that are required to explain the large scatter in opacities. We discuss the implications of this scenario for cosmological reionization. Finally, we investigate whether other statistics applied to the z > 5 Ly α forest can shed light on the origin of the scatter. Compared to a model with a uniform ionizing background, models that successfully account for the scatter lead to enhanced power in the line-of-sight flux power spectrum on scales k ≲0.1 h Mpc^-1. We find tentative evidence for this enhancement in observations of the high-redshift Ly α forest.