Spectroscopic confirmation of a mature galaxy cluster at a redshift of 2


Galaxy clusters are the most massive virialized structures in the Universe and are formed through the gravitational accretion of matter over cosmic time1. The discovery2 of an evolved galaxy cluster at redshift z = 2, corresponding to a look-back time of 10.4 billion years, provides an opportunity to study its properties. The galaxy cluster XLSSC 122 was originally detected as a faint, extended X-ray source in the XMM Large Scale Structure survey and was revealed to be coincident with a compact over-density of galaxies2 with photometric redshifts of 1.9 ± 0.2. Subsequent observations3 at millimetre wavelengths detected a Sunyaev–Zel’dovich decrement along the line of sight to XLSSC 122, thus confirming the existence of hot intracluster gas, while deep imaging spectroscopy from the European Space Agency’s X-ray Multi-Mirror Mission (XMM-Newton) revealed4 an extended, X-ray-bright gaseous atmosphere with a virial temperature of 60 million Kelvin, enriched with metals to the same extent as are local clusters. Here we report optical spectroscopic observations of XLSSC 122 and identify 37 member galaxies at a mean redshift of 1.98, corresponding to a look-back time of 10.4 billion years. We use photometry to determine a mean, dust-free stellar age of 2.98 billion years, indicating that star formation commenced in these galaxies at a mean redshift of 12, when the Universe was only 370 million years old. The full range of inferred formation redshifts, including the effects of dust, covers the interval from 7 to 13. These observations confirm that XLSSC 122 is a remarkably mature galaxy cluster with both evolved stellar populations in the member galaxies and a hot, metal-rich gas composing the intracluster medium.

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Fig. 1: HST image of the galaxy cluster XLSSC 122.
Fig. 2: Redshift histogram of all galaxies along the line of sight to XLSSC 122.
Fig. 3: Colour–magnitude diagram of all galaxies within the HST/WFC3 field of view.
Fig. 4: The luminosity-weighted age distribution of stars within red-sequence cluster galaxies.

Data availability

All HST data presented in this paper are publicly available at the Hubble Legacy Archive (https://hla.stsci.edu/). The programme number is 15267.


  1. 1.

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

  2. 2.

    Willis, J. P. et al. Distant galaxy clusters in the XMM Large Scale Structure survey. Mon. Not. R. Astron. Soc. 430, 134–156 (2013).

  3. 3.

    Mantz, A. B. et al. The XXL survey. V. Detection of the Sunyaev–Zel’dovich effect of the redshift 1.9 galaxy cluster XLSSU J021744.1–034536 with CARMA. Astrophys. J. 794, 157 (2014).

  4. 4.

    Mantz, A. B. et al. The XXL survey. XVII. X-ray and Sunyaev–Zel’dovich properties of the redshift 2.0 galaxy cluster XLSSC 122. Astron. Astrophys. 620, A2 (2018).

  5. 5.

    Bower, R. G., Lucey, J. R. & Ellis, R. S. Precision photometry of early-type galaxies in the Coma and Virgo clusters: a test of the universality of the colour–magnitude relation. II. Analysis. Mon. Not. R. Astron. Soc. 254, 601–613 (1992).

  6. 6.

    Hashimoto, T. et al. The onset of star formation 250 million years after the Big Bang. Nature 557, 392–395 (2018).

  7. 7.

    Sarazin, C. L. X-Ray Emission From Galaxy Clusters (Cambridge Univ. Press, 1988).

  8. 8.

    Roettiger, K., Stone, J. M. & Mushotzky, R. F. Anatomy of a merger: a numerical model of A754. Astrophys. J. 493, 62–72 (1998).

  9. 9.

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

  10. 10.

    Harrison, I. & Hotchkiss, S. A consistent approach to falsifying ΛCDM with rare galaxy clusters. J. Cosmol. Astropart. Phys. 7, 022 (2013).

  11. 11.

    Gavazzi, R. et al. A weak lensing study of the Coma cluster. Astron. Astrophys. 498, L33–L36 (2009).

  12. 12.

    Miller, T. B. et al. A massive core for a cluster of galaxies at a redshift of 4.3. Nature 556, 469–472 (2018).

  13. 13.

    Bennett, C. L., Larson, D., Weiland, J. L. & Hinshaw, G. The 1% concordance Hubble constant. Astrophys. J. 794, 135 (2014).

  14. 14.

    Brammer, G. GRIZLI: grism redshift and line analysis software. Astrophys. Source Code Library record ascl:1905.001 (2019).

  15. 15.

    Brammer, G., Pirzkal, N., McCullough, P. & MacKenty, J. Time-Varying Excess Earth-Glow Backgrounds In The WFC3/IR Channel. Instrument Science Report WFC3 2014-03 (Space Telescope Science Institute, 2014).

  16. 16.

    Brammer, G. Reprocessing WFC3/IR Exposures Affected by Time-Variable Backgrounds. Instrument Science Report WFC3 2016-16 (Space Telescope Science Institute, 2016).

  17. 17.

    Stanford, S. A. et al. IDCS J1426.5+3508: discovery of a massive, infrared-selected galaxy cluster at z = 1.75. Astrophys. J. 753, 164 (2012).

  18. 18.

    Kron, R. G. Photometry of a complete sample of faint galaxies. Astrophys. J. Suppl. Ser. 43, 305–325 (1980).

  19. 19.

    Horne, K. An optimal extraction algorithm for CCD spectroscopy. Publ. Astron. Soc. Pacif. 98, 609–617 (1986).

  20. 20.

    Brammer, G. B., van Dokkum, P. G. & Coppi, P. EAZY: A fast, public photometric redshift code. Astrophys. J. 686, 1503–1513 (2008).

  21. 21.

    Ruel, J. et al. Optical spectroscopy and velocity dispersions of galaxy clusters from the SPT-SZ Survey. Astrophys. J. 792, 45 (2014).

  22. 22.

    Stanford, S. A., Eisenhardt, P. R. & Dickinson, M. The evolution of early-type galaxies in distant clusters. Astrophys. J. 492, 461–479 (1998).

  23. 23.

    Gunn, J. E. & Gott, J. R. III On the infall of matter into clusters of galaxies and some effects on their evolution. Astrophys. J. 176, 1–19 (1972).

  24. 24.

    Butcher, H. & Oemler, A. Jr The evolution of galaxies in clusters. II. The galaxy content of nearby clusters. Astrophys. J. 226, 559–565 (1978).

  25. 25.

    Jung, S. et al. On the origin of gas-poor galaxies in galaxy clusters using cosmological hydrodynamic simulations. Astrophys. J. 865, 156 (2018).

  26. 26.

    Andreon, S. et al. JKCS 041: a Coma cluster progenitor at z = 1.803. Astron. Astrophys. 565, A120 (2014).

  27. 27.

    Newman, A. B. et al. Spectroscopic confirmation of the rich z = 1.80 galaxy cluster JKCS 041 using the WFC3 grism: environmental trends in the ages and structure of quiescent galaxies. Astrophys. J. 788, 51 (2014).

  28. 28.

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

  29. 29.

    Salpeter, E. E. The luminosity function and stellar evolution. Astrophys. J. 121, 161–167 (1955).

  30. 30.

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

  31. 31.

    Longhetti, M. et al. Dating the stellar population in massive early-type galaxies at z ~ 1.5. Mon. Not. R. Astron. Soc. 361, 897–906 (2005).

  32. 32.

    Strazzullo, V. et al. The red sequence at birth in the galaxy cluster Cl J1449+0856 at z = 2. Astrophys. J. 833, L20 (2016).

  33. 33.

    Gobat, R. et al. WFC3 grism confirmation of the distant cluster Cl J1449+0856 at <z> = 2.00: quiescent and star-forming galaxy populations. Astrophys. J. 776, 9 (2013).

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We acknowledge the builders of the XMM-LSS and XXL surveys, on whose work this paper is based. This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with programme number 15267. J.P.W. and E.S.N. acknowledge support from NSERC. R.E.A.C., E.S.N., S.W.A., A.L.K., A.M. and R.G.M. acknowledge support from NASA grant number HST-GO-15267.002-A. The Cosmic Dawn Center is funded by the Danish National Research Foundation.

Author information

J.P.W., R.E.A.C. and E.S.N. analysed the data and wrote the paper. S.W.A., A.L.K., A.M., R.G.M., S.A.S. and G.B. provided guidance on the analysis and commented on the paper.

Correspondence to J. P. Willis.

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The authors declare no competing interests.

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Peer review information Nature thanks Florence Durret and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Example spectra of two member galaxies of XLSSC 122.

a, The brightest cluster galaxy (ID 526) as the black line with error bars with the best-fitting, redshifted galaxy template shown in red (see Methods). b, A fainter cluster member with strong emission lines (ID 1141) as the black line with error bars. Error bars indicate the 1-sigma measurement uncertainty. The vertical dashed lines show the observed frame location of [O ii] 3,727 Å, Hβ 4,861 Å and [O iii] 5,007 Å at a redshift of 1.963.

Extended Data Fig. 2 The luminosity-weighted stellar age versus the mass of red sequence cluster member galaxies.

Posterior distributions in mean stellar age (tw) and log stellar mass for the 19 ‘gold’ members of the cluster red sequence. Only SED models assuming AV = 0.0 are shown. Contours enclose 67% of the posterior probability for each galaxy. The horizontal dashed line indicates an age of 3.35 Gyr, that is, the age of the Universe at a redshift z = 1.98 in the assumed cosmological model.

Extended Data Fig. 3 The luminosity-weighted stellar age distributions for red-sequence cluster member galaxies.

Panels show posterior distributions in tw for each ‘gold’ member galaxy of XLSSC 122, having marginalized over Mstar. For convenience, the same colour scheme is employed as in Extended Data Fig. 2. In each panel the solid, dashed and dotted curves display, respectively, SED models characterized by AV = 0.0, 0.3 and 0.5. The vertical dashed line in each panel indicates the age of the Universe at z = 1.98.

Extended Data Table 1 Measured properties of confirmed members of XLSSC 122 and z = 1.93 structure

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Willis, J.P., Canning, R.E.A., Noordeh, E.S. et al. Spectroscopic confirmation of a mature galaxy cluster at a redshift of 2. Nature 577, 39–41 (2020). https://doi.org/10.1038/s41586-019-1829-4

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