Galaxy clusters trace the largest structures of the Universe and provide ideal laboratories for studying galaxy evolution and cosmology1,2. Clusters with extended X-ray emission have been discovered at redshifts of up to z ≈ 2.5 (refs 3,4,5,6,7). Meanwhile, there has been growing interest in hunting for protoclusters, the progenitors of clusters, at higher redshifts8,9,10,11,12,13,14. It is, however, very challenging to find the largest protoclusters at early times, when they start to assemble. Here, we report a giant protocluster of galaxies at z ≈ 5.7, when the Universe was only one billion years old. This protocluster occupies a volume of about 353 cubic comoving megaparsecs. It is embedded in an even larger overdense region with at least 41 spectroscopically confirmed, luminous Lyα-emitting galaxies (Lyα emitters, or LAEs), including several previously reported LAEs9. Its LAE density is 6.6 times the average density at z ≈ 5.7. It is the only one of its kind in an LAE survey in 4 deg2 on the sky. Such a large structure is also rarely seen in current cosmological simulations. This protocluster will collapse into a galaxy cluster with a mass of (3.6 ± 0.9) × 1015 solar masses, comparable to those of the most massive clusters or protoclusters known so far.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Additional information

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


  1. 1.

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

  2. 2.

    Kravtsov, A. V. & Borgani, S. Formation of galaxy clusters. Ann. Rev. Astron. Astrophys. 50, 353–409 (2012).

  3. 3.

    Papovich, C. et al. A Spitzer-selected galaxy cluster at z = 1.62. Astrophys. J. 716, 1503–1513 (2010).

  4. 4.

    Gobat, R. et al. A mature cluster with X-ray emission at z = 2.07.Astron. Astrophys. 526, 133–145 (2011).

  5. 5.

    Stanford, S. A. et al. Discovery of a massive, infrared-selected galaxy cluster at z = 1.75. Astrophys. J. 753, 164–171 (2012).

  6. 6.

    Andreon, S. et al. JKCS041: a Coma cluster progenitor at z = 1.803.Astron. Astrophys. 565, 120–134 (2014).

  7. 7.

    Wang, T. et al. Discovery of a galaxy cluster with a violently starbursting core at z = 2.506. Astrophys. J. 828, 56–70 (2016).

  8. 8.

    Steidel, C. C. et al. A large structure of galaxies at redshift z ~ 3 and its cosmological implications. Astrophys. J. 492, 428–438 (1998).

  9. 9.

    Ouchi, M. et al. The discovery of primeval large-scale structures with forming clusters at redshift 6. Astrophys. J. Lett. 620, 1–4 (2005).

  10. 10.

    Venemans, B. P. et al. Protoclusters associated with z > 2 radio galaxies—I. Characteristics of high redshift protoclusters.Astron. Astrophys. 461, 823–845 (2007).

  11. 11.

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

  12. 12.

    Toshikawa, J. et al. Discovery of a protocluster at z ~ 6. Astrophys. J. 750, 137–148 (2012).

  13. 13.

    Dey, A. et al. Spectroscopic confirmation of a protocluster at z ≈ 3.786. Astrophys. J. 823, 11–28 (2016).

  14. 14.

    Franck, J. R. & McGaugh, S. S. The Candidate Cluster and Protocluster Catalog (CCPC)-II. Spectroscopically identified structures spanning 2 <z<6.6. Astrophys. J. 833, 15–33 (2016).

  15. 15.

    Overzier, R. A. et al. ΛCDM predictions for galaxy protoclusters—I. The relation between galaxies, protoclusters and quasars at z ~ 6. Mon. Not. R. Astron. Soc. 394, 577–594 (2009).

  16. 16.

    Chiang, Y.-S., Overzier, R. & Gebhardt, K. Ancient light from young cosmic cities: physical and observational signatures of galaxy protoclusters. Astrophys. J. 779, 127–142 (2013).

  17. 17.

    Furusawa, H. & et al. The Subaru XMM-NEWTON Deep Survey (SXDS)—II. Optical imaging and photometric catalogs.Astrophys. J.Suppl. 176, 1–18 (2008).

  18. 18.

    Rhoads, J. E. et al. A luminous Lyα-emitting galaxy at redshift z = 6.535: discovery and spectroscopic confirmation. Astrophys. J. 611, 59–67 (2004).

  19. 19.

    Shimasaku, K. et al. Lyà emitters at z=5.7 in the Subaru Deep Field. Publ. Astron. Soc. Jpn 58, 313–334 (2006).

  20. 20.

    Hu, E. M. et al. An atlas of z = 5.7 and z = 6.5 Lyα emitters. Astrophys. J. 725, 394–423 (2010).

  21. 21.

    Kashikawa, N. et al. Completing the census of Lyα emitters at the reionization epoch. Astrophys. J. 734, 119–137 (2011).

  22. 22.

    Mateo, M. et al. M2FS: the Michigan/Magellan Fiber System. Proc. SPIE 8446, 84464Y (2012).

  23. 23.

    Jiang, L. et al. A Magellan M2FS spectroscopic survey of galaxies at 5.5 < z < 6.8: program overview and a sample of the brightest Lyα emitters. Astrophys. J. 846, 134–148 (2017).

  24. 24.

    Henriques, B. M. B. et al. Galaxy formation in the Planck cosmology—I. Matching the observed evolution of star formation rates, colours and stellar masses. Mon. Not. R. Astron. Soc. 451, 2663–2680 (2015).

  25. 25.

    Menanteau, F. et al. The Atacama Cosmology Telescope: ACT-CL J0102–4915 ‘El Gordo’, a massive merging cluster at redshift 0.87. Astrophys. J. 748, 7–24 (2012).

  26. 26.

    Casey, C. M. et al. A massive, distant proto-cluster at z = 2.47 caught in a phase of rapid formation. Astrophys. J. Lett. 808, 33–40 (2015).

  27. 27.

    Cai, Z. et al. MApping the Most Massive Overdensities Through Hydrogen (MAMMOTH)-II. Discovery of the extremely massive overdensity BOSS1441 at z=2.32. Astrophys. J. 839, 131–141 (2017).

  28. 28.

    Chiang, Y.-K. et al. Galaxy protoclusters as drivers of cosmic star-formation history in the first 2 Gyr. Astrophys. J. Lett. 844, 23–29 (2017).

  29. 29.

    Wyithe, J. S. B. & Loeb, A. A characteristic size of ~10 Mpc for the ionized bubbles at the end of cosmic reionization. Nature 432, 194–196 (2004).

  30. 30.

    Iliev, I. T. et al. Simulating cosmic reionization at large scales—I. The geometry of reionization. Mon. Not. R. Astron. Soc. 369, 1625–1638 (2006).

  31. 31.

    Jiang, L. et al. Physical properties of spectroscopically confirmed galaxies at z≥6—I. Basic characteristics of the rest-frame UV continuum and Lyα emission. Astrophys. J. 772, 99–118 (2013).

  32. 32.

    Ouchi, M. et al. Systematic Identification of LAEs for Visible Exploration and Reionization Research Using Subaru HSC (SILVERRUSH)—I. Program strategy and clustering properties of ~2,000 Lyα emitters at z=6–7 over the 0.3–0.5 Gpc2 survey area.Publ. Astron. Soc. Jpn 70, S13 (2018).

Download references


We acknowledge support from the National Key R&D Program of China (2016YFA0400703 and 2016YFA0400702) and from the National Science Foundation of China (grant 11533001). G.A.B. is supported by CONICYT/FONDECYT, Programa de Iniciacion, Folio 11150220. E.W.O. acknowledges support from the NSF from grant AST1313006. We thank R. de Grijs and M. Ouchi for discussions. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. Australian access to the Magellan Telescopes was supported through the National Collaborative Research Infrastructure Strategy of the Australian Federal Government.

Author information


  1. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China

    • Linhua Jiang
    • , Jin Wu
    • , Luis C. Ho
    • , Ran Wang
    •  & Xue-Bing Wu
  2. Department of Astronomy, School of Physics, Peking University, Beijing, China

    • Jin Wu
    • , Luis C. Ho
    •  & Xue-Bing Wu
  3. Research School of Astronomy and Astrophysics, Australian National University, Weston Creek, Australian Capital Territory, Australia

    • Fuyan Bian
  4. Department of Physics & Astronomy, The Johns Hopkins University, Baltimore, MD, USA

    • Yi-Kuan Chiang
  5. Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL, USA

    • Yue Shen
  6. National Centre for Supercomputing Applications, University of Illinois at Urbana-Champaign, Urbana, IL, USA

    • Yue Shen
  7. CAS Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Shanghai, China

    • Zhen-Ya Zheng
  8. Institute of Astrophysics and Center for Astroengineering, Pontificia Universidad Catolica de Chile, Santiago, Chile

    • Zhen-Ya Zheng
  9. Chinese Academy of Sciences South America Center for Astronomy, Santiago, Chile

    • Zhen-Ya Zheng
  10. Department of Astronomy, University of Michigan, Ann Arbor, MI, USA

    • John I. Bailey III
    •  & Mario Mateo
  11. Leiden Observatory, Leiden University, Leiden, The Netherlands

    • John I. Bailey III
  12. Observatories of the Carnegie Institution for Science, Pasadena, CA, USA

    • Guillermo A. Blanc
    •  & Jeffrey D. Crane
  13. Departamento de Astronomía, Universidad de Chile, Santiago, Chile

    • Guillermo A. Blanc
    •  & Grecco A. Oyarzún
  14. Steward Observatory, University of Arizona, Tucson, AZ, USA

    • Xiaohui Fan
    •  & Edward W. Olszewski


  1. Search for Linhua Jiang in:

  2. Search for Jin Wu in:

  3. Search for Fuyan Bian in:

  4. Search for Yi-Kuan Chiang in:

  5. Search for Luis C. Ho in:

  6. Search for Yue Shen in:

  7. Search for Zhen-Ya Zheng in:

  8. Search for John I. Bailey III in:

  9. Search for Guillermo A. Blanc in:

  10. Search for Jeffrey D. Crane in:

  11. Search for Xiaohui Fan in:

  12. Search for Mario Mateo in:

  13. Search for Edward W. Olszewski in:

  14. Search for Grecco A. Oyarzún in:

  15. Search for Ran Wang in:

  16. Search for Xue-Bing Wu in:


L.J. is the Principal Investigator of the project, analysed the data and prepared the manuscript. J.W. reduced the M2FS images. F.B., Y.S., Z.-Y.Z., J.I.B., J.D.C., M.M. and E.W.O. helped with the M2FS observations. Y.-K.C. carried out the simulations. L.C.H., X.F., R.W. and X.-B.W. prepared the manuscript. G.A.B. and G.A.O. helped with the M2FS data reduction. All authors helped with the scientific interpretations and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Linhua Jiang.

Supplementary information

  1. Supplementary Information

    Supplementary Table 1, Supplementary Figure 1

About this article

Publication history