According to the current understanding of cosmic structure formation, the precursors of the most massive structures in the Universe began to form shortly after the Big Bang, in regions corresponding to the largest fluctuations in the cosmic density field1,2,3. Observing these structures during their period of active growth and assembly—the first few hundred million years of the Universe—is challenging because it requires surveys that are sensitive enough to detect the distant galaxies that act as signposts for these structures and wide enough to capture the rarest objects. As a result, very few such objects have been detected so far4,5. Here we report observations of a far-infrared-luminous object at redshift 6.900 (less than 800 million years after the Big Bang) that was discovered in a wide-field survey6. High-resolution imaging shows it to be a pair of extremely massive star-forming galaxies. The larger is forming stars at a rate of 2,900 solar masses per year, contains 270 billion solar masses of gas and 2.5 billion solar masses of dust, and is more massive than any other known object at a redshift of more than 6. Its rapid star formation is probably triggered by its companion galaxy at a projected separation of 8 kiloparsecs. This merging companion hosts 35 billion solar masses of stars and has a star-formation rate of 540 solar masses per year, but has an order of magnitude less gas and dust than its neighbour and physical conditions akin to those observed in lower-metallicity galaxies in the nearby Universe7. These objects suggest the presence of a dark-matter halo with a mass of more than 100 billion solar masses, making it among the rarest dark-matter haloes that should exist in the Universe at this epoch.

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ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This work incorporates observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI) operated by AURA. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. The SPT is supported by the NSF through grant PLR-1248097, with partial support through PHY-1125897, the Kavli Foundation and the Gordon and Betty Moore Foundation grant GBMF 947. Supporting observations were obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, under a cooperative agreement with the NSF on behalf of the Gemini partnership of NSF (USA), NRC (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina) and Ministério da Ciência, Tecnologia e Inovação (Brazil). D.P.M., J.S.S., J.D.V., K.C.L. and J.S. acknowledge support from the US NSF under grant AST-1312950. D.P.M. was partially supported by NASA through grant HST-GO-14740 from the Space Telescope Science Institute. K.C.L. was partially supported by SOSPA4-007 from the National Radio Astronomy Observatory. The Flatiron Institute is supported by the Simons Foundation. J.D.V. acknowledges support from an A. P. Sloan Foundation Fellowship. Y.D.H. is a Hubble fellow.

Author information


  1. Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, Arizona 85721, USA

    • D. P. Marrone
    • , J. S. Spilker
    • , K. C. Litke
    •  & M. Tang
  2. Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, New York 10010, USA

    • C. C. Hayward
  3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

    • C. C. Hayward
    • , M. L. N. Ashby
    •  & A. A. Stark
  4. Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, Illinois 61801, USA

    • J. D. Vieira
    • , S. Lower
    • , K. A. Phadke
    •  & J. Sreevani
  5. Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Avenida Ejército 441, Santiago, Chile

    • M. Aravena
  6. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • M. B. Bayliss
  7. Aix Marseille Université, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France

    • M. Béthermin
  8. Department of Physics and Astronomy, University of Missouri, 5110 Rockhill Road, Kansas City, Missouri 64110, USA

    • M. Brodwin
  9. Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK

    • M. S. Bothwell
  10. Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

    • M. S. Bothwell
  11. Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA

    • J. E. Carlstrom
    •  & T. M. Crawford
  12. Department of Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA

    • J. E. Carlstrom
  13. Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA

    • J. E. Carlstrom
  14. Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA

    • J. E. Carlstrom
    •  & T. M. Crawford
  15. Dalhousie University, Halifax, Nova Scotia, Canada

    • S. C. Chapman
    • , D. J. M. Cunningham
    • , T. B. Miller
    •  & K. M. Rotermund
  16. European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany

    • Chian-Chou Chen
    •  & C. De Breuck
  17. Department of Astronomy and Physics, Saint Mary’s University, Halifax, Nova Scotia, Canada

    • D. J. M. Cunningham
  18. Department of Physics, University of California, One Shields Avenue, Davis, California 95616, USA

    • C. D. Fassnacht
  19. Department of Astronomy, University of Florida, Bryant Space Sciences Center, Gainesville, Florida 32611 USA

    • A. H. Gonzalez
    • , J. Ma
    •  & D. Narayanan
  20. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

    • T. R. Greve
  21. Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, California 94305, USA

    • Y. D. Hezaveh
    •  & W. R. Morningstar
  22. Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada

    • K. Lacaille
  23. Department of Physics and Astronomy, University of California, Los Angeles, California 90095-1547, USA

    • M. Malkan
  24. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22903, USA

    • E. J. Murphy
  25. Large Synoptic Survey Telescope, 950 North Cherry Avenue, Tucson, Arizona 85719, USA

    • B. Stalder
  26. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

    • M. L. Strandet
    •  & A. Weiß
  27. International Max Planck Research School (IMPRS) for Astronomy and Astrophysics, Universities of Bonn and Cologne, Bonn, Germany

    • M. L. Strandet


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D.P.M. proposed the ALMA [C ii] and [O iii] line observations and analysed all ALMA data. J.S.S. performed the lens modelling. C.C.H. led the rareness analysis. M.L.N.A., M.B.B., S.C.C., A.H.G., J.M., K.M.R. and B.S. provided optical and infrared data reduction and de-convolution. K.A.P. and J.D.V. performed SED modelling of the sources and lens. A.W. performed joint dust and line modelling of high-redshift targets. D.P.M. wrote the manuscript. J.S.S., C.C.H., D.P.M., S.L., K.A.P. and J.D.V. prepared the figures. All authors discussed the results and provided comments on the paper. Authors are ordered alphabetically after J.D.V.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to D. P. Marrone.

Reviewer Information Nature thanks R. Davé and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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