Abstract
We present evidence that a region of high effective Ly$\alpha$ optical depth
at $z\sim5.7$ is associated with an underdense region at the tail end of cosmic
reionization. We carried out a survey of Lyman Break Galaxies (LBGs) using
Subaru Hyper Suprime-Cam (HSC) in the field of the $z=5.98$ quasar J0148+0600,
whose spectrum presents an unusually long ($\sim160 cMpc$) and highly
opaque ($\gtrsim7$) Ly$\alpha$ trough at $5.5z5.9$. LBG candidates
were selected to lie within the redshift range of the trough, and the projected
number densities were measured within 90 cMpc of the quasar sightline. The
region within $8'$ (or, $\approx19 cMpc$) of the quasar position is
the most underdense of the whole field. This is consistent with the significant
deficit of Ly$\alpha$-emitters (LAE) at $z=5.72$ reported by Becker et al., and
suggests that the paucity of LAEs is not purely due to removal of the
Ly$\alpha$ emission by the high opacity but reflects a real coherent
underdensity of galaxies across the entire redshift range of the trough. These
observations are consistent with scenarios in which large optical depth
fluctuations arise due to fluctuations in the galaxy-dominant UV background or
due to residual neutral islands that are expected from reionization that is
completed at redshifts as low as $złesssim5.5$.
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