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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.

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Data availability

All HST data presented in this paper are publicly available at the Hubble Legacy Archive ( The programme number is 15267.


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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.

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Authors and Affiliations



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.

Corresponding author

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.

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

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).

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