A closer look at using quasar near-zones as a probe of neutral hydrogen in the intergalactic medium

We examine a large set of synthetic quasar spectra to realistically assess the potential of using the relative sizes of highly ionized near-zones in the Ly alpha and Ly beta forest as a probe of the neutral hydrogen content of the intergalactic medium (IGM) at z > 6. The scatter in the relative near-zone size distribution, induced by underlying fluctuations in the baryonic density field and the filtering of ionizing radiation, is considerable even for fixed assumptions about the IGM neutral fraction. As a consequence, the current observational data cannot distinguish between an IGM which is significantly neutral or one which is highly ionized just above z = 6. Under standard assumptions for quasar ages and ionizing luminosities, a future sample of several tens of high-resolution Lya and Ly beta near-zone spectra should be capable of distinguishing between a volume-weighted neutral hydrogen fraction in the IGM which is greater or less than 10 per cent.