2013MNRAS.428.3058S

The spectra of several high-redshift (z > 6) quasars have shown indications for a Gunn-Peterson (GP) damping wing, suggesting a substantial mean neutral hydrogen fraction ({bar}x _H \scriptscriptstyle I^IGM\gtrsim 0.03) in the z ~ 6 intergalactic medium (IGM). However, previous analyses assumed that the IGM was uniformly ionized outside of the quasar's H ii region. Here we relax this assumption and model patchy reionization scenarios for a range of IGM and quasar parameters. Compared to uniform reionization, patchy reionization imprints a different average damping wing profile with an associated sightline-to-sightline scatter. We quantify the impact of these differences on the inferred {bar}x _H \scriptscriptstyle I^IGM, by fitting the spectra of three quasars: SDSS J1148+5251 (z = 6.4189), J1030+0524 (z = 6.308) and J1623+3112 (z = 6.247). We find that the best-fitting values of {bar}x _H \scriptscriptstyle I^IGM in the patchy models agree well with the uniform case. More importantly, we confirm that the observed spectra favour the presence of a GP damping wing, with peak likelihoods decreasing by factors of ≳ few-10 when the spectra are modelled without a damping wing. We also find that the Lyα absorption spectra, by themselves, cannot distinguish the damping wing in a relatively neutral IGM from a damping wing in a highly ionized IGM, caused either by an isolated neutral patch or by a damped Lyα absorber (DLA). However, neutral patches in a highly ionized universe ({bar}x _H \scriptscriptstyle I^IGM\lesssim 10^-2) and DLAs with the large required column densities (N_H I few×10^20 cm^-2) are both rare. As a result, when we include reasonable prior probabilities for the line of sight (LOS) to intercept either a neutral patch or a DLA at the required distance of ∼ 40-60 comoving Mpc away from the quasar, we find strong lower limits on the neutral fraction in the IGM, {bar}x _H \scriptscriptstyle I^IGM\gtrsim 0.1 (at 95 per cent confidence). This supports earlier claims that a substantial global fraction of hydrogen in the z ~ 6 IGM is in neutral form.