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


Massive clusters of galaxies have been found that date from as early as 3.9 billion years1 (3.9 Gyr; z = 1.62) after the Big Bang, containing stars that formed at even earlier epochs2,3. 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 cluster4,5. 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 starbursts4,5,6. Observational evidence for this picture, however, is sparse because high-redshift protoclusters are rare and difficult to observe6,7. 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 gas8. These massive galaxies place a lower limit of more than 4 × 1011 solar masses of dark and luminous matter in this region, consistent with that expected from cosmological simulations for the earliest galaxy clusters4,5,7.

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Figure 1: Spectra of confirmed cluster members.
Figure 2: Image of the region around the protocluster core.
Figure 3: Detail of the protocluster core.


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These results are based on observations with: the W. M. Keck Observatory, the IRAM Plateau de Bure Interferometer, the IRAM 30-m telescope with the GISMO 2-mm camera, the Chandra X-ray Observatory, the Subaru Telescope, the Hubble Space Telescope, the Canada-France-Hawaii Telescope with WIRCam and MegaPrime, the United Kingdom Infrared Telescope, the Spitzer Space Telescope, the Smithsonian Submillimeter Array Telescope, the James Clerk Maxwell Telescope with the AzTEC 1.1mm camera, and the National Radio Astronomy Observatory’s Very Large Array. D.R. and B.R. acknowledge support from NASA through Hubble Fellowship grants awarded by the Space Telescope Science Institute. P.L.C. and N.Z.S. acknowledge grant support from NASA. G.W.W., M.Y. and J.G.S. acknowledge grant support from the NSF.

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Authors and Affiliations



P.L.C. led the spectroscopic effort, reduced the spectroscopic and photometric data, and led the scientific analysis including the optical and radio/millimetre fitting analysis and cluster properties. N.Z.S. led the spectroscopic and photometric follow-up efforts. D.R., C.C., P.C. and R.N. assisted with the physical interpretation of the radio data. B.R. provided cosmological simulations to check the significance of the protocluster and the likelihood of finding it. M.S., L.Y., M.E., F.C. and B.M. carried out the Keck observations and assisted with the data reduction. E.S. reduced and analysed the radio data. G.W.W. and M.Y. assisted with the submillimetre data analysis. F.C. and M.E. assisted with the X-ray data analysis. A.K. coordinated the 2-mm observations. J.G.S. conducted and reduced the 2-mm observations.

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Correspondence to Peter L. Capak.

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The authors declare no competing financial interests.

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Capak, P., Riechers, D., Scoville, N. et al. A massive protocluster of galaxies at a redshift of z ≈ 5.3. Nature 470, 233–235 (2011).

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