2017MNRAS.466.4239G

We quantify the presence of Lyα damping wing absorption from a partially neutral intergalactic medium (IGM) in the spectrum of the z = 7.08 QSO, ULASJ1120+0641. Using a Bayesian framework, we simultaneously account for uncertainties in: (i) the intrinsic QSO emission spectrum; and (ii) the distribution of cosmic H I patches during the epoch of reionization (EoR). For (i), we use a new intrinsic Lyα emission line reconstruction method, sampling a covariance matrix of emission line properties built from a large data base of moderate-z QSOs. For (ii), we use the Evolution of 21-cm Structure (EOS; Mesinger et al.) simulations, which span a range of physically motivated EoR models. We find strong evidence for the presence of damping wing absorption redward of Lyα (where there is no contamination from the Lyα forest). Our analysis implies that the EoR is not yet complete by z = 7.1, with the volume-weighted IGM neutral fraction constrained to {bar}x_ H I_ = 0.40^+0.21 ^_ -0.19_ at 1σ ({bar}x_ H I_ = 0.40^+0.41 ^_ -0.32_ at 2σ). This result is insensitive to the EoR morphology. Our detection of significant neutral H I in the IGM at z = 7.1 is consistent with the latest Planck 2016 measurements of the CMB Thompson scattering optical depth.