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

• Published in: Nature (London) Vol. 577; no. 7788; pp. 39 - 41
• Main Authors: Willis, J. P., Canning, R. E. A., Noordeh, E. S., Allen, S. W., King, A. L., Mantz, A., Morris, R. G., Stanford, S. A., Brammer, G.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 02.01.2020; Nature Publishing Group
• Subjects: 639/33/34/124; 639/33/34/863; Age; Astronomical spectroscopy; ASTRONOMY AND ASTROPHYSICS; Clusters; Compact galaxies; Deposition; Dust; Dust effects; Galactic clusters; Galactic evolution; Galaxies; Gravity; Humanities and Social Sciences; Large scale structure of the universe; Metals; Methods; multidisciplinary; Photometry; Red shift; Science; Science (multidisciplinary); Simulation; Spectroscopy; Spectrum analysis; Star & galaxy formation; Star formation; Stars & galaxies; Stellar age; Stellar evolution; Stellar populations; Universe; Wavelengths; X ray sources; XMM (spacecraft); United Kingdom
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-019-1829-4

Galaxy clusters are the most massive virialized structures in the Universe and are formed through the gravitational accretion of matter over cosmic time 1 . The discovery 2 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 galaxies 2 with photometric redshifts of 1.9 ± 0.2. Subsequent observations 3 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) revealed 4 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. The redshift of a seemingly mature galaxy cluster has been confirmed spectroscopically to be about 2 and photometry indicates that star formation began in the galaxies of the cluster at a redshift of about 12.