Ly alpha emissters at z=6.5 in the SSA22 field: An area more neutral or void at the end of the reionization epoch

TL;DR

This study surveys Lyman alpha emitters at z=6.5 in the SSA22 field, finding a lower density than average, suggesting the field may be a rare void with implications for the neutral hydrogen fraction during reionization.

Contribution

It provides the first detailed comparison of LAE density in SSA22 with other fields at z=6.5, indicating possible large-scale structure effects on reionization studies.

Findings

SSA22 field has a 3 times lower LAE density than the average at z=6.5.

The neutral hydrogen fraction in SSA22 may be up to twice that of the cosmic average.

Large survey areas are needed for robust constraints on reionization.

Abstract

We present results of a survey of Lyman $\alpha$ emitters (LAEs) at $z=6.5$ which is thought to be the final epoch of the cosmic reionization. In a $\approx530$ arcmin$^2$ deep image of the SSA22 field taken through a narrowband filter NB912 installed in the Subaru/Suprime-Cam, we have found only 14 LAE candidates with $L_{\rm Ly\alpha}\ga3\times10^{42}$ erg s$^{-1}$. Even applying the same colour selection criteria, the number density of the LAE candidates is a factor of 3 smaller than that found at the same redshift in the Subaru Deep field (SDF). Assuming the number density in the SDF is a cosmic average, the probability to have a number density equal to or smaller than that found in the SSA22 field is only 7% if we consider fluctuation by the large-scale structure (i.e. cosmic variance) and Poisson error. Therefore, the SSA22 field may be a rare void at $z=6.5$. On the other hand, we have found that the number density of $i'$-drop galaxies with $25.5<z'<26.0$ in the SSA22 field agrees well with that in the SDF. If we consider a scenario that a larger neutral fraction of intergalactic hydrogen, $x_{\rm HI}$, in the SSA22 field obscures a part of Ly$\alpha$ emission, $x_{\rm HI}$ in the SSA22 field should be about 2 times larger than that in the SDF. This can be translated into $x_{\rm HI}<0.9$ at $z=6.5$ in the SSA22 field. A much larger survey area than previous ones is required to overcome a large fluctuation reported here and to obtain a robust constraint on $x_{\rm HI}$ at the end of the reionization from LAEs.