Onset of Cosmic Reionization: Evidence of An Ionized Bubble Merely 680
  Myrs after the Big Bang
  While most of the inter-galactic medium (IGM) today is permeated by ionized
hydrogen, it was largely filled with neutral hydrogen for the first 700 million
years after the Big Bang. The process that ionized the IGM (cosmic
reionization) is expected to be spatially inhomogeneous, with fainter galaxies
playing a significant role. However, we still have only a few direct
constraints on the reionization process. Here we report the first spectroscopic
confirmation 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. The
physical separation among the three members is < 0.7 Mpc. We estimate the
radius of ionized bubble of the brightest galaxy to be about 1.02 Mpc, and show
that the individual ionized bubbles formed by all three galaxies likely overlap
significantly, forming a large yet localized ionized region, which leads to the
spatial inhomogeneity in the reionization process. It is striking that two of
three galaxies in EGS77 are quite faint in the continuum, thanks to our
selection of reionizing sources using their Lyman-alpha line emission. Indeed,
one is the faintest spectroscopically confirmed galaxy yet discovered at such
high redshifts. Our observations provide direct constraints in the process of
cosmic reionization, and allow us to investigate the properties of sources
responsible for reionizing the universe.
