Constraining Reionization in Progress at z=5.7 with Lyman-$\alpha$ Emitters: Voids, Peaks, and Cosmic Variance

TL;DR

This paper investigates how the distribution of Lyman-alpha emitters at redshifts 5.7 and 6.6 can constrain the progress of cosmic reionization, emphasizing the utility of void probability functions over traditional correlation measures.

Contribution

It introduces models of LAE distributions during reionization, demonstrating the effectiveness of void probability functions in constraining the neutral fraction of the intergalactic medium.

Findings

Void probability function is more sensitive than 2PCF during late reionization.

Models show higher neutral fractions increase the likelihood of voids and peaks in LAE distribution.

Cosmic variance observed is consistent with large-scale structure in a ΛCDM universe.

Abstract

A number of independent observations suggest that the intergalactic medium was significantly neutral at $z=7$ and that reionization was, perhaps, still in progress at $z=5.7$. The narrowband survey, SILVERRUSH, has mapped over $2,000$ Lyman-$\alpha$ emitters (LAEs) at these redshifts. Previous analyses have assumed that reionization was over by $z=5.7$, but this data may actually sample the final stages of reionization when the last neutral islands were relegated to the cosmic voids. Motivated by these developments, we reexamine LAE void and peak statistics and their ability to constrain reionization. We construct models of the LAE distribution in (1 Gpc$/h$)$^3$ volumes, spanning a range of neutral fractions at $z=5.7$ and 6.6. Models with a higher neutral fraction show an enhanced probability of finding holes in the LAE distribution. When comparing models at fixed mean surface density, however, LAEs obscured by neutral gas in the voids must be compensated by visible LAEs elsewhere. Hence, in these models the likelihood of finding an over-dense peak is also enhanced in the latter half of reionization. Compared to the widely used angular two-point correlation function (2PCF), we find that the void probability function (VPF) provides a more sensitive test of models during the latter half of reionization. By comparison, at neutral fractions $\sim 50\%$, the VPF and a simple peak thresholding statistic are both similar to the 2PCF in constraining power. Lastly, we find that the cosmic variance and large-scale asymmetries observed in the SILVERRUSH fields are consistent with large-scale structure in a $\Lambda$CDM universe.