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

We report the discovery of an extremely long (∼110 Mpc h−1) and dark (τeff ≳ 7) Lyα trough extending down to z ≃ 5.5 towards the zem ≃ 6.0 quasar ULAS J0148+0600. We use these new data in combination with Lyα forest measurements from 42 quasars at 4.5 ≤ zem ≤ 6.4 to conduct an updated analysis of the line-of-sight variance in the intergalactic Lyα opacity over 4 ≤ z ≤ 6. We find that the scatter in transmission among lines of sight near z ∼ 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 ≃ 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 ≃ 5.6, however, a large fraction of the data are consistent with a uniform UVB, and by z ∼ 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 ∼ 6, it has fully completed by z ∼ 5.