Massive clusters of galaxies have been found as early as 3.9 billion years after the Big Bang. Cosmological simulations predict that these systems should descend from 'protoclusters' — early overdensities of massive galaxies that merge hierarchically to form a cluster. Observational evidence for this picture, however, is sparse because high-redshift protoclusters are rare and difficult to observe. Here, a protocluster region 1 billion years (z = 5.3) after the Big Bang is reported. This cluster extends over >13 megaparsecs, contains a luminous quasar as well as a system rich in molecular gas. A lower limit of >4 × 1011 solar masses of dark and luminous matter in this region is placed, consistent with that expected from cosmological simulations.
- Peter L. Capak
- Dominik Riechers
- Johannes G. Staguhn
