Publication Date:
2013-01-03
Description:
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 ( $\bar{x}_{\rm H\,{\scriptscriptstyle I}}^{\rm IGM}\gtrsim 0.03$ ) 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 $\bar{x}_{\rm H\,{\scriptscriptstyle I}}^{\rm IGM}$ , 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 $\bar{x}_{\rm H\,{\scriptscriptstyle I}}^{\rm IGM}$ 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 ( $\bar{x}_{\rm H\,{\scriptscriptstyle I}}^{\rm IGM}\lesssim 10^{-2}$ ) and DLAs with the large required column densities ( $N_{\rm H\,{\small I}}\gtrsim {\rm { few}}\times 10^{20}{\rm { cm}^{-2}}$ ) 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, $\bar{x}_{\rm H\,{\scriptscriptstyle I}}^{\rm IGM}\gtrsim 0.1$ (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.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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