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We perform a ground-based near-infrared spectroscopic survey using theKeck/MOSFIRE spectrograph to target Ly$\alpha$ emission at $7.0<z<8.2$ from 61galaxies to trace the ionization state of the intergalactic medium (IGM). Wecover a total effective sky area of $\sim10^\prime\times10^\prime$ in theExtended Groth Strip field of the Cosmic Assembly Near-infrared DeepExtragalactic Legacy Survey. From our observations, we detect Ly$\alpha$emission at a $>$4$\sigma$ level in eight $z>7$ galaxies, which includeadditional members of the known $z\sim7.7$ Ly$\alpha$-emitter (LAE) cluster(Tilvi et al. 2020). With the addition of these newly-discovered $z\sim7.7$LAEs, this is currently the largest measured LAE cluster at $z>7$. Theunusually-high Ly$\alpha$ detection rate at $z\sim7.7$ in this field suggestssignificantly stronger Ly$\alpha$ emission from the clustered LAEs than fromthe rest of our targets. We estimate the ionized bubble sizes around these LAEsand conclude that the LAEs are clustered within an extended ionized structurecreated by overlapping ionized bubbles which allow the easier escape ofLy$\alpha$ from galaxies. It is remarkable that the brightest object in thecluster has the lowest measured redshift of the Ly$\alpha$ line, being placedin front of the other LAEs in the line-of-sight direction. This suggests thatwe are witnessing the enhanced IGM transmission of Ly$\alpha$ from galaxies onthe rear side of an ionized area. This could be a consequence of Ly$\alpha$radiative transfer: Ly$\alpha$ close to the central velocity is substantiallyscattered by the IGM while Ly$\alpha$ from the rear-side galaxies issignificantly redshifted to where it has a clear path.

New $z > 7$ Lyman-alpha Emitters in EGS: Evidence of an Extended Ionized Structure at $z \sim 7.7$