A massive protocluster of galaxies at a redshift of z ≈ 5.3

An early look at a galactic cluster A 'protocluster' of massive galaxies at a redshift z = 5.3, dating to only a billion years after the Big Bang, has been discovered in data from the Cosmological Evolution Survey (COSMOS), a project combining the power of the Hubble Space Telescope and ground-based telescopes on 2 square degrees of sky in the constellation Sextans. The protocluster occupies an overdense region more than 13 megaparsecs (40 million light years) across, rich in molecular gas and young stars. Its properties match the predictions of galaxy formation simulations, suggesting that the protocluster will evolve into a massive galaxy cluster typical of those seen at lower redshifts. 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 × 10 11 solar masses of dark and luminous matter in this region is placed, consistent with that expected from cosmological simulations. Massive clusters of galaxies have been found that date from as early as 3.9 billion years 1 (3.9 Gyr; z = 1.62) after the Big Bang, containing stars that formed at even earlier epochs 2 , 3 . Cosmological simulations using the current cold dark matter model predict that these systems should descend from ‘protoclusters’—early overdensities of massive galaxies that merge hierarchically to form a cluster 4 , 5 . These protocluster regions themselves are built up hierarchically and so are expected to contain extremely massive galaxies that can be observed as luminous quasars and starbursts 4 , 5 , 6 . Observational evidence for this picture, however, is sparse because high-redshift protoclusters are rare and difficult to observe 6 , 7 . Here we report a protocluster region that dates from 1 Gyr ( z = 5.3) after the Big Bang. This cluster of massive galaxies extends over more than 13 megaparsecs and contains a luminous quasar as well as a system rich in molecular gas 8 . These massive galaxies place a lower limit of more than 4 × 10