Evidence for a Highly Opaque Large-scale Galaxy Void at the End of Reionization

We present evidence that a region of high effective Lya optical depth at z similar to 5.7 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 in the field of the z=5.98 quasar J0148+0600, whose spectrum presents an unusually long (similar to 160 cMpc) and opaque (t greater than or similar to 7) Lya 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 similar to 19 cMpc) of the quasar position is the most underdense of the whole field. The significance of the presence of the void is estimated to be 99%. This is consistent with the significant deficit of Lya emitters (LAEs) at z=5.72 reported by Becker et al. and suggests that the paucity of LAEs is not purely due to the removal of the Lya 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 z less than or similar to 5.5.