CHORUS IV: Mapping the Spatially Inhomogeneous Cosmic Reionization with
  Subaru HSC
  The spatial inhomogeneity is one of the important features for understanding
the reionization process; however, it has not yet been fully quantified. To map
this inhomogeneous distribution, we simultaneously detect Ly$\alpha$ emitters
(LAEs) and Lyman break galaxies (LBGs) at $z \sim 6.6$ from the Subaru/Hyper
Suprime-Cam (HSC) large-area ($\sim1.5\,\mathrm{ deg}^2 =
34000\,\mathrm{cMpc}^2$) deep survey. We estimate the neutral fraction,
$x_\mathrm{HI}$, from the observed number density ratio of LAEs to LBGs,
$n(\mathrm{LAE})/n(\mathrm{LBG})$ based on numerical radiative transfer
simulation, in which model galaxies are selected to satisfy the observed
selection function. While the average $x_\mathrm{HI}$ within the field of view
is found to be $x_\mathrm{HI} < 0.4$, which is consistent with previous
studies, the variation of $n(\mathrm{LAE})/n(\mathrm{LBG})$ within the field of
view for each $140\,\mathrm{pMpc}^2$ is found to be as large as a factor of
three. This may suggest a spatially inhomogeneous topology of reionization, but
it also leaves open the possibility that the variation is based on the inherent
large-scale structure of the galaxy distribution. Based on the simulations, it
may be difficult to distinguish between the two from the current survey. We
also find that LAEs in the high LAE density region are more populate high
$\mathrm{EW}_0$, supporting that the observed $n(\mathrm{LAE})/n(\mathrm{LBG})$
is more or less driven by the neutral fraction, though the statistical
significance is not high.
