Evidence of patchy hydrogen reionization from an extreme Lyα trough below redshift six

We report the discovery of an extremely long (similar to 110 Mpc h(-1)) and dark (tau(eff) greater than or similar to 7) Ly alpha trough extending down to z similar or equal to 5.5 towards the z(em) similar or equal to 6.0 quasar ULAS J0148+0600. We use these new data in combination with Ly alpha forest measurements from 42 quasars at 4.5 <= z(em) <= 6.4 to conduct an updated analysis of the line-of-sight variance in the intergalactic Ly alpha opacity over 4 <= z <= 6. We find that the scatter in transmission among lines of sight near z similar to 6 significantly exceeds theoretical expectations for either a uniform ultraviolet background (UVB) or simple fluctuating UVB models in which the mean free path to ionizing photons is spatially invariant. The data, particularly near z similar or equal to 5.6-5.8, instead require fluctuations in the volume-weighted hydrogen neutral fraction that are a factor of 3 or more beyond those expected from density variations alone. We argue that these fluctuations are most likely driven by large-scale variations in the mean free path, consistent with expectations for the final stages of inhomogeneous hydrogen reionization. Even by z similar or equal to 5.6, however, a large fraction of the data are consistent with a uniform UVB, and by z similar to 5 the data are fully consistent with opacity fluctuations arising solely from the density field. This suggests that while reionization may be ongoing at z similar to 6, it has fully completed by z similar to 5.