Large fluctuations in the high-redshift metagalactic ionizing background

Recent observations have shown that the scatter in opacities amongst coeval segments of the Ly alpha 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 approximate to 5.5 requires a short spatially averaged mean free path of less than or similar to 15 h(-1) comoving Mpc, a factor of greater than or similar to 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 HI photoionization rate, which stay approximately constant over the interval z approximate to 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 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 alpha 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 less than or similar to 0.1 h Mpc(-1). We find tentative evidence for this enhancement in observations of the high-redshift Ly alpha forest.