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

TL;DR

This study uses synthetic quasar spectra to evaluate how effectively near-zone sizes in Lya and Lyb forests can reveal the neutral hydrogen fraction in the IGM at z>6, highlighting current limitations and future potential.

Contribution

It provides a realistic assessment of near-zone size variability and its implications for measuring IGM neutrality at high redshift.

Findings

Current data cannot distinguish IGM neutrality levels just above z=6.

A future sample of tens of high-resolution spectra could differentiate between neutral fractions above or below 10%.

Significant scatter in near-zone sizes is caused by density fluctuations and radiation filtering.

Abstract

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 Lya and Lyb 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 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 Lyb 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.