New z > 7 Lyman-alpha Emitters in EGS: Evidence of an Extended Ionized Structure at z 7. 7

We perform a ground-based near-infrared spectroscopic survey using the Keck/MOSFIRE spectrograph to target Lyα emission at 7. 04σ level in eight z>7 galaxies, which include additional members of the known z7. 7 Lyα-emitter (LAE) cluster (Tilvi et al. 2020). With the addition of these newly-discovered z7. 7 LAEs, this is currently the largest measured LAE cluster at z>7. The unusually-high Lyα detection rate at z7. 7 in this field suggests significantly stronger Lyα emission from the clustered LAEs than from the rest of our targets. We estimate the ionized bubble sizes around these LAEs and conclude that the LAEs are clustered within an extended ionized structure created by overlapping ionized bubbles which allow the easier escape of Lyα from galaxies. It is remarkable that the brightest object in the cluster has the lowest measured redshift of the Lyα line, being placed in front of the other LAEs in the line-of-sight direction. This suggests that we are witnessing the enhanced IGM transmission of Lyα from galaxies on the rear side of an ionized area. This could be a consequence of Lyα radiative transfer: Lyα close to the central velocity is substantially scattered by the IGM while Lyα from the rear-side galaxies is significantly redshifted to where it has a clear path.