Evidence of patchy hydrogen reionization from an extreme Lyα trough below redshift six
Becker, George D. and Bolton, James S. and Madau, Piero and Pettini, Max and Ryan-Weber, Emma V. and Venemans, Bram P. (2015) Evidence of patchy hydrogen reionization from an extreme Lyα trough below redshift six. Monthly Notices of the Royal Astronomical Society, 447 (4). pp. 3402-3419. ISSN 1365-2966
Official URL: http://mnras.oxfordjournals.org/content/447/4/3402
We report the discovery of an extremely long (∼110 Mpc/h) 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.
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