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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1.

    et al. Simulations of the formation, evolution and clustering of galaxies and quasars. Nature 435, 629–636 (2005)

  2. 2.

    , , & The statistical properties of Λ cold dark matter halo formation. Mon. Not. R. Astron. Soc. 383, 546–556 (2008)

  3. 3.

    , & The average star formation histories of galaxies in dark matter halos from z = 0–8. Astrophys. J. 770, 57 (2013)

  4. 4.

    et al. A dust-obscured massive maximum-starburst galaxy at a redshift of 6.34. Nature 496, 329–333 (2013)

  5. 5.

    et al. Dusty starburst galaxies in the early Universe as revealed by gravitational lensing. Nature 495, 344–347 (2013)

  6. 6.

    et al. ISM properties of a massive dusty star-forming galaxy discovered at z 7. Astrophys. J. 842, L15 (2017)

  7. 7.

    et al. The Herschel dwarf galaxy survey. I. Properties of the low-metallicity ISM from PACS spectroscopy. Astron. Astrophys. 578, A53 (2015)

  8. 8.

    et al. The 10 meter South Pole telescope. Publ. Astron. Soc. Pacif. 123, 568–581 (2011)

  9. 9.

    . et al. Extragalactic millimeter-wave point-source catalog, number counts and statistics from 771 deg2 of the SPT-SZ survey. Astrophys. J. 779, 61 (2013)

  10. 10.

    Planck Collaboration. Planck 2015 results. XIII. Cosmological parameters. Astron. Astrophys. 594, A13 (2016)

  11. 11.

    , & Dusty star-forming galaxies at high redshift. Phys. Rep. 541, 45–161 (2014)

  12. 12.

    et al. Detection of lensing substructure using ALMA observations of the dusty galaxy SDP.81. Astrophys. J. 823, 37 (2016)

  13. 13.

    & Gasdynamics and starbursts in major mergers. Astrophys. J. 464, 641–663 (1996)

  14. 14.

    et al. HerMES: the rest-frame UV emission and a lensing model for the z = 6.34 luminous dusty starburst galaxy HFLS3. Astrophys. J. 790, 40 (2014)

  15. 15.

    et al. Rapidly star-forming galaxies adjacent to quasars at redshifts exceeding 6. Nature 545, 457–461 (2017)

  16. 16.

    et al. Near-infrared MOSFIRE spectra of dusty star-forming galaxies at 0.2 < z < 4. Astrophys. J. 840, 101 (2017)

  17. 17.

    et al. UV luminosity functions at redshifts z 4 to z 10: 10,000 galaxies from HST legacy fields. Astrophys. J. 803, 34 (2015)

  18. 18.

    et al. First detection of the [O III] 88 μm line at high redshifts: characterizing the starburst and narrow-line regions in extreme luminosity systems. Astrophys. J. 714, L147–L151 (2010)

  19. 19.

    , & A compendium of far-infrared line and continuum emission for 227 galaxies observed by the Infrared Space Observatory. Astrophys. J. Suppl. Ser. 178, 280–301 (2008)

  20. 20.

    et al. Detection of an oxygen emission line from a high-redshift galaxy in the reionization epoch. Science 352, 1559–1562 (2016)

  21. 21.

    et al. Physical conditions in the gas phases of the giant H II region LMC-N 11 unveiled by Herschel. I. Diffuse [C II] and [O III] emission in LMC-N 11B. Astron. Astrophys. 548, A91 (2012)

  22. 22.

    et al. Explaining the [C II]157.7 μm deficit in luminous infrared galaxies—first results from a Herschel/PACS study of the GOALS sample. Astrophys. J. 774, 68 (2013)

  23. 23.

    et al. Witnessing the birth of the red sequence: ALMA high-resolution imaging of [C II] and dust in two interacting ultra-red starbursts at z = 4.425. Astrophys. J. 827, 34 (2016)

  24. 24.

    et al. ALMA imaging and gravitational lens models of South Pole telescope—selected dusty, star-forming galaxies at high redshifts. Astrophys. J. 826, 112 (2016)

  25. 25.

    , & The CO-to-H2 conversion factor. Annu. Rev. Astron. Astrophys. 51, 207–268 (2013)

  26. 26.

    et al. A survey of molecular gas in luminous sub-millimetre galaxies. Mon. Not. R. Astron. Soc. 429, 3047–3067 (2013)

  27. 27.

    et al. A survey of the cold molecular gas in gravitationally lensed star-forming galaxies at z > 2. Mon. Not. R. Astron. Soc. 457, 4406–4420 (2016)

  28. 28.

    & A consistent approach to falsifying ΛCDM with rare galaxy clusters. J. Cosmology Astropart. Phys. 7, 022 (2013)

  29. 29.

    et al. Highly-excited CO emission in APM 08279+5255 at z = 3.9. Astron. Astrophys. 467, 955–969 (2007)

  30. 30.

    et al. ALMA redshifts of millimeter-selected galaxies from the SPT survey: the redshift distribution of dusty star-forming galaxies. Astrophys. J. 767, 88 (2013)

  31. 31.

    et al. The redshift distribution of dusty star-forming galaxies from the SPT survey. Astrophys. J. 822, 80 (2016)

  32. 32.

    et al. ALMA observations of SPT-discovered, strongly lensed, dusty, star-forming galaxies. Astrophys. J. 767, 132 (2013)

  33. 33.

    et al. The Infrared Array Camera (IRAC) for the Spitzer space telescope. Astrophys. J. Suppl. Ser. 154, 10–17 (2004)

  34. 34.

    et al. SEDS: the Spitzer extended deep survey. Survey design, photometry, and deep IRAC source counts. Astrophys. J. 769, 80 (2013)

  35. 35.

    , & IRACproc: a software suite for processing and analyzing Spitzer/IRAC data. Proc. SPIE 6270, 627020 (2006)

  36. 36.

    & SExtractor: software for source extraction. Astron. Astrophys. Suppl. Ser. 117, 393–404 (1996)

  37. 37.

    et al. The Gemini-North multi-object spectrograph: performance in imaging, long-slit, and multi-object spectroscopic modes. Publ. Astron. Soc. Pacif. 116, 425–440 (2004)

  38. 38.

    et al. FLAMINGOS-2: the facility near-infrared wide-field imager and multi-object spectrograph for Gemini. Proc. SPIE 8446, 84460I (2012)

  39. 39.

    ., ., & GPUs and Python: a recipe for lightning-fast data pipelines. ASP Conf. Ser. 461, 53–56 (2012)

  40. 40.

    ., ., & Redefining the data pipeline using GPUs. ASP Conf. Ser. 475, 79–82 (2013)

  41. 41.

    et al. Stellar masses and star formation rates of lensed, dusty, star-forming galaxies from the SPT survey. Astrophys. J. 812, 88 (2015)

  42. 42.

    , , & A Bayesian analysis of regularized source inversions in gravitational lensing. Mon. Not. R. Astron. Soc. 371, 983–998 (2006)

  43. 43.

    , & EAZY: a fast, public photometric redshift code. Astrophys. J. 686, 1503–1513 (2008)

  44. 44.

    , & Star formation and dust attenuation properties in galaxies from a statistical ultraviolet-to-far-infrared analysis. Mon. Not. R. Astron. Soc. 360, 1413–1425 (2005)

  45. 45.

    et al. Analysis of galaxy spectral energy distributions from far-UV to far-IR with CIGALE: studying a SINGS test sample. Astron. Astrophys. 507, 1793–1813 (2009)

  46. 46.

    & Stellar population synthesis at the resolution of 2003. Mon. Not. R. Astron. Soc. 344, 1000–1028 (2003)

  47. 47.

    Galactic stellar and substellar initial mass function. Publ. Astron. Soc. Pacif. 115, 763–795 (2003)

  48. 48.

    & Infrared emission from interstellar dust. IV. The silicate-graphite-PAH model in the post-Spitzer era. Astrophys. J. 657, 810–837 (2007)

  49. 49.

    et al. The dust content and opacity of actively star-forming galaxies. Astrophys. J. 533, 682–695 (2000)

  50. 50.

    , , & Global far-ultraviolet (912–1800 Å) properties of star-forming galaxies. Astrophys. J. Suppl. Ser. 140, 303–329 (2002)

  51. 51.

    , & Submillimetre and far-infrared spectral energy distributions of galaxies: the luminosity-temperature relation and consequences for photometric redshifts. Mon. Not. R. Astron. Soc. 338, 733–744 (2003)

  52. 52.

    et al. Spitzer imaging of herschel-atlas gravitationally lensed submillimeter sources. Astrophys. J. 728, L4 (2011)

  53. 53.

    et al. The complex physics of dusty star-forming galaxies at high redshifts as revealed by Herschel and Spitzer. Astrophys. J. 762, 108 (2013)

  54. 54.

    , & The propagation of uncertainties in stellar population synthesis modeling. I. The relevance of uncertain aspects of stellar evolution and the initial mass function to the derived physical properties of galaxies. Astrophys. J. 699, 486–506 (2009)

  55. 55.

    & The propagation of uncertainties in stellar population synthesis modeling. III. Model calibration, comparison, and evaluation. Astrophys. J. 712, 833–857 (2010)

  56. 56.

    et al. CO (2–1) line emission in redshift 6 quasar host galaxies. Astrophys. J. 739, L34 (2011)

  57. 57.

    et al. Far-infrared and molecular CO emission from the host galaxies of faint quasars at z 6. Astron. J. 142, 101 (2011)

  58. 58.

    et al. The intense starburst HDF 850.1 in a galaxy overdensity at z ≈ 5.2 in the Hubble Deep Field. Nature 486, 233–236 (2012)

  59. 59.

    et al. A dusty, normal galaxy in the epoch of reionization. Nature 519, 327–330 (2015)

  60. 60.

    et al. Bright [C II] and dust emission in three z > 6.6 quasar host galaxies observed by ALMA. Astrophys. J. 816, 37 (2016)

  61. 61.

    et al. The compact, 1 kpc host galaxy of a quasar at a redshift of 7.1. Astrophys. J. 837, 146 (2017)

  62. 62.

    et al. Dust in the reionization era: ALMA observations of a z = 8.38 gravitationally lensed galaxy. Astrophys. J. 837, L21 (2017)

  63. 63.

    et al. Molecular gas in the host galaxy of a quasar at redshift z = 6.42. Nature 424, 406–408 (2003)

  64. 64.

    et al. A massive molecular gas reservoir in the z = 5.3 submillimeter galaxy AzTEC-3. Astrophys. J. 720, L131–L136 (2010)

  65. 65.

    et al. Molecular gas in z 6 quasar host galaxies. Astrophys. J. 714, 699–712 (2010)

  66. 66.

    et al. Star formation and gas kinematics of quasar host galaxies at z 6: new insights from ALMA. Astrophys. J. 773, 44 (2013)

  67. 67.

    et al. [C II] and 12CO(1–0) emission maps in HLSJ091828.6+514223: a strongly lensed interacting system at z = 5.24. Astrophys. J. 783, 59 (2014)

  68. 68.

    & Molecular gas at high redshift. Annu. Rev. Astron. Astrophys. 43, 677–725 (2005)

  69. 69.

    & Cool gas in high-redshift galaxies. Annu. Rev. Astron. Astrophys. 51, 105–161 (2013)

  70. 70.

    & How stellar feedback simultaneously regulates star formation and drives outflows. Mon. Not. R. Astron. Soc. 465, 1682–1698 (2017)

  71. 71.

    et al. Toward a halo mass function for precision cosmology: the limits of universality. Astrophys. J. 688, 709–728 (2008)

  72. 72.

    & Effects of strong gravitational lensing on millimeter-wave galaxy number counts. Astrophys. J. 734, 52–59 (2011)

  73. 73.

    , , & The bias of the submillimetre galaxy population: SMGs are poor tracers of the most-massive structures in the z 2 Universe. Mon. Not. R. Astron. Soc. 452, 878–883 (2015)

Download references


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


  1. Search for D. P. Marrone in:

  2. Search for J. S. Spilker in:

  3. Search for C. C. Hayward in:

  4. Search for J. D. Vieira in:

  5. Search for M. Aravena in:

  6. Search for M. L. N. Ashby in:

  7. Search for M. B. Bayliss in:

  8. Search for M. Béthermin in:

  9. Search for M. Brodwin in:

  10. Search for M. S. Bothwell in:

  11. Search for J. E. Carlstrom in:

  12. Search for S. C. Chapman in:

  13. Search for Chian-Chou Chen in:

  14. Search for T. M. Crawford in:

  15. Search for D. J. M. Cunningham in:

  16. Search for C. De Breuck in:

  17. Search for C. D. Fassnacht in:

  18. Search for A. H. Gonzalez in:

  19. Search for T. R. Greve in:

  20. Search for Y. D. Hezaveh in:

  21. Search for K. Lacaille in:

  22. Search for K. C. Litke in:

  23. Search for S. Lower in:

  24. Search for J. Ma in:

  25. Search for M. Malkan in:

  26. Search for T. B. Miller in:

  27. Search for W. R. Morningstar in:

  28. Search for E. J. Murphy in:

  29. Search for D. Narayanan in:

  30. Search for K. A. Phadke in:

  31. Search for K. M. Rotermund in:

  32. Search for J. Sreevani in:

  33. Search for B. Stalder in:

  34. Search for A. A. Stark in:

  35. Search for M. L. Strandet in:

  36. Search for M. Tang in:

  37. Search for A. Weiß in:


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.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

About this article

Publication history







By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.