Abstract
We report the discovery of an extremely long ($\sim$110 Mpc/$h$) and dark
($\tau_eff 7$) Ly$\alpha$ trough extending down to $z 5.5$
towards the $z_em 6.0$ 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 $z 6$
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 three 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 is 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 $z \sim
5$.
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