Lyα emitters at z= 6.5 in the SSA22 field: an area more neutral or void at the end of the re-ionization epoch★

Abstract

We present results of a survey of Lyman $\\alpha$ emitters (LAEs) at $z=6.5$\nwhich is thought to be the final epoch of the cosmic reionization. In a\n$\\approx530$ arcmin$^2$ deep image of the SSA22 field taken through a\nnarrowband filter NB912 installed in the Subaru/Suprime-Cam, we have found only\n14 LAE candidates with $L_{\\rm Ly\\alpha}\\ga3\\times10^{42}$ erg s$^{-1}$. Even\napplying the same colour selection criteria, the number density of the LAE\ncandidates is a factor of 3 smaller than that found at the same redshift in the\nSubaru Deep field (SDF). Assuming the number density in the SDF is a cosmic\naverage, the probability to have a number density equal to or smaller than that\nfound in the SSA22 field is only 7% if we consider fluctuation by the\nlarge-scale structure (i.e. cosmic variance) and Poisson error. Therefore, the\nSSA22 field may be a rare void at $z=6.5$. On the other hand, we have found\nthat the number density of $i'$-drop galaxies with $25.5<z'<26.0$ in the SSA22\nfield agrees well with that in the SDF. If we consider a scenario that a larger\nneutral fraction of intergalactic hydrogen, $x_{\\rm HI}$, in the SSA22 field\nobscures a part of Ly$\\alpha$ emission, $x_{\\rm HI}$ in the SSA22 field should\nbe about 2 times larger than that in the SDF. This can be translated into\n$x_{\\rm HI}<0.9$ at $z=6.5$ in the SSA22 field. A much larger survey area than\nprevious ones is required to overcome a large fluctuation reported here and to\nobtain a robust constraint on $x_{\\rm HI}$ at the end of the reionization from\nLAEs.\n