http://arxiv.org/abs/1407.4850
We report the discovery of an extremely long ($\sim$110 Mpc/$h$) and dark ($\tau_{\rm eff} \gtrsim 7$) Ly$\alpha$ trough extending down to $z \simeq 5.5$ towards the $z_{\rm em} \simeq 6.0$ quasar ULAS J0148+0600. We use these new data in combination with Ly$\alpha$ forest measurements from 42 quasars at $4.5 \le z_{\rm em} \le 6.4$ to conduct an updated analysis of the line-of-sight variance in the intergalactic Ly$\alpha$ opacity over $4 \le z \le 6$. We find that the scatter in transmission among lines of sight near $z \sim 6$ 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 $z \simeq 5.6$-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 $z \simeq 5.6$, however, a large fraction of the data is consistent with a uniform UVB, and by $z \sim 5$ the data are fully consistent with opacity fluctuations arising solely from the density field. This suggests that while reionization may be ongoing at $z \sim 6$, it has fully completed by $z \sim 5$.
G. Becker, J. Bolton, P. Madau, et. al.
Mon, 21 Jul 14
9/55
Comments: 19 pages, 20 figures, submitted to MNRAS, comments welcome
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