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While most of the inter-galactic medium (IGM) today is permeated by ionizedhydrogen, it was largely filled with neutral hydrogen for the first 700 millionyears after the Big Bang. The process that ionized the IGM (cosmicreionization) is expected to be spatially inhomogeneous, with fainter galaxiesplaying a significant role. However, we still have only a few directconstraints on the reionization process. Here we report the first spectroscopicconfirmation of two galaxies and very likely a third galaxy in a group(hereafter EGS77) at redshift z = 7.7, merely 680 Myrs after the Big Bang. Thephysical separation among the three members is < 0.7 Mpc. We estimate theradius of ionized bubble of the brightest galaxy to be about 1.02 Mpc, and showthat the individual ionized bubbles formed by all three galaxies likely overlapsignificantly, forming a large yet localized ionized region, which leads to thespatial inhomogeneity in the reionization process. It is striking that two ofthree galaxies in EGS77 are quite faint in the continuum, thanks to ourselection of reionizing sources using their Lyman-alpha line emission. Indeed,one is the faintest spectroscopically confirmed galaxy yet discovered at suchhigh redshifts. Our observations provide direct constraints in the process ofcosmic reionization, and allow us to investigate the properties of sourcesresponsible for reionizing the universe.

Onset of Cosmic Reionization: Evidence of An Ionized Bubble Merely 680 Myrs after the Big Bang