A Lyman-{\alpha} protocluster at redshift 6.9
  Protoclusters, the progenitors of the most massive structures in the
Universe, have been identified at redshifts of up to 6.6. Besides exploring
early structure formation, searching for protoclusters at even higher redshifts
is particularly useful to probe the reionization. Here we report the discovery
of the protocluster LAGER-z7OD1 at a redshift of 6.93, when the Universe was
only 770 million years old and could be experiencing rapid evolution of the
neutral hydrogen fraction in the intergalactic medium. The protocluster is
identified by an overdensity of 6 times the average galaxy density, and with 21
narrowband selected Lyman-$\alpha$ galaxies, among which 16 have been
spectroscopically confirmed. At redshifts similar to or above this record,
smaller protogroups with fewer members have been reported. LAGER-z7OD1 shows an
elongated shape and consists of two subprotoclusters, which would have merged
into one massive cluster with a present-day mass of $3.7 \times 10^{15}$ solar
masses. The total volume of the ionized bubbles generated by its member
galaxies is found to be comparable to the volume of the protocluster itself,
indicating that we are witnessing the merging of the individual bubbles and
that the intergalactic medium within the protocluster is almost fully ionized.
LAGER-z7OD1 thus provides a unique natural laboratory to investigate the
reionization process.
