Evidence of Gunn-Peterson damping wings in high-z quasar spectra: strengthening the case for incomplete reionization at z ∼ 6-7

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 ((x) over bar (IGM)(HI) greater than or similar to 0.03) in the z approximate to 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 (x) over bar (IGM)(HI), 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 (x) over bar (IGM)(HI) 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 greater than or similar to few-10 when the spectra are modelled without a damping wing. We also find that the Ly alpha 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 Ly alpha absorber (DLA). However, neutral patches in a highly ionized universe ((x) over bar (IGM)(HI) less than or similar to 10(-2)) and DLAs with the large required column densities (N-HI greater than or similar to few x 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 similar to 40-60 comoving Mpc away from the quasar, we find strong lower limits on the neutral fraction in the IGM, (x) over bar (IGM)(HI) greater than or similar to 0.1 (at 95 per cent confidence). This supports earlier claims that a substantial global fraction of hydrogen in the z approximate to 6 IGM is in neutral form.