Model-independent evidence in favor of an end to reionization by z~6

TL;DR

This study provides model-independent upper limits on the neutral hydrogen fraction at redshifts 5-6, strongly indicating that reionization was nearly complete by z~6, based on dark pixel analysis of quasar spectra.

Contribution

It updates previous dark pixel analysis with a larger quasar sample, offering the most stringent model-independent constraints on reionization completion at z~6.

Findings

Upper limit on <xHI> at z=5.9 is 0.06 ± 0.05

Upper limit on <xHI> at z=5.6 is 0.04 ± 0.05

Reionization is likely complete by z~6

Abstract

We present new upper limits on the volume-weighted neutral hydrogen fraction, <xHI>, at z~5-6 derived from spectroscopy of bright quasars. The fraction of the Lyman-alpha and Lyman-beta forests that is "dark" (with zero flux) provides the only model-independent upper limit on <xHI>, requiring no assumptions about the physical conditions in the intergalactic medium or the quasar's unabsorbed UV continuum. In this work we update our previous results using a larger sample (22 objects) of medium-depth (~ few hours) spectra of high-redshift quasars obtained with the Magellan, MMT, and VLT. This significantly improves the upper bound on <xHI> derived from dark pixel analysis to <xHI> <= 0.06 + 0.05 (1{\sigma}) at z=5.9, and <xHI> <= 0.04 + 0.05 (1{\sigma}) at z=5.6. These results provide robust constraints for theoretical models of reionization, and provide the strongest available evidence that reionization has completed (or is very nearly complete) by z~6.