Was reionization complete by z ~ 5-6?

TL;DR

This paper investigates whether reionization was complete by redshift 5-6, highlighting the observational and modeling challenges, and proposing methods to better constrain the neutral hydrogen fraction during this epoch.

Contribution

It demonstrates the difficulty of constraining reionization completeness at z~5-6 and proposes model-independent methods using dark pixel fractions and dark gap distributions.

Findings

Ruling out incomplete reionization at z~6 is challenging due to biased quasar environments.

Residual neutral hydrogen absorption dominates over neutral regions in the IGM.

Current quasar samples are insufficient to constrain the neutral fraction to better than 10% at z~6.

Abstract

It is generally taken for granted that reionization has completed by z=6, due to the detection of flux in the Lyman alpha forest at redshifts z<6. However, since reionization is expected to be highly inhomogeneous, much of the spectra pass through just the ionized component of the intergalactic medium (IGM) even for non-negligible values of the volume-weighted mean neutral hydrogen fraction, x_HI. We study the expected signature of an incomplete reionization at z ~ 5--6, using very large-scale (2 Gpc) seminumeric simulations. We find that ruling out an incomplete reionization is difficult at these redshifts since: (1) quasars reside in biased regions of the ionization field, with fewer surrounding HI patches than implied by the global mean, x_H; (2) absorption from the residual neutral hydrogen inside the ionized IGM generally dominates over the absorption from the remaining HI regions; (3) modeling the Lyman alpha forest and its redshift evolution even in just the ionized IGM is very difficult, and nearly impossible to do a priori. We propose using the fraction of pixels which are dark as a simple, nearly model-independent upper limit on x_HI. Alternately, the size distribution of regions with no detectable flux (dark gaps) can be used to place a more model dependent constraint. Either way, the current sample of quasars is statistically insufficient to constrain x_HI at z~6 to even the 10 per cent level. At z~5, where there are more available sightlines and the forest is less dark, constraining x_HI < 0.1 might be possible given a large dynamic range from very deep spectra and/or the Lyman beta forest. We conclude with the caution against over-interpreting the observations. There is currently no direct evidence that reionization was complete by z ~ 5-6.