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