1. Leiden Observatory, University of Leiden, PO Box 9513, Leiden, 2300 RA, The Netherlands
2. Department of Physics & Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
3. Lick Observatory, University of California, Santa Cruz, California 95064, USA
4. Space Telescope Science Institute, Baltimore, Maryland 21218, USA
5. European Southern Observatory, Garching, D-85748, Germany
6. Bartko Science & Technology, Mead, Colorado 80542-0670, USA
7. The Racah Institute of Physics, Hebrew University, Jerusalem, 91904, Israel
8. NASA-Goddard Space Flight Centre, Greenbelt, Maryland 20771, USA
9. Departmento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile

Correspondence to: George K. Miley¹ Email: miley@strw.leidenuniv.nl

Abstract

The most massive galaxies and the richest clusters are believed to have emerged from regions with the largest enhancements of mass density^{1, 2, 3, 4} relative to the surrounding space. Distant radio galaxies may pinpoint the locations of the ancestors of rich clusters, because they are massive systems associated with 'overdensities' of galaxies that are bright in the Lyman- line of hydrogen^{5, 6, 7}. A powerful technique for detecting high-redshift galaxies is to search for the characteristic 'Lyman break' feature in the galaxy colour, at wavelengths just shortwards of Ly, which is due to absorption of radiation from the galaxy by the intervening intergalactic medium. Here we report multicolour imaging of the most distant candidate^{7, 8, 9} protocluster, TN J1338–1942 at a redshift z 4.1. We find a large number of objects with the characteristic colours of galaxies at that redshift, and we show that this excess is concentrated around the targeted dominant radio galaxy. Our data therefore indicate that TN J1338–1942 is indeed the most distant cluster progenitor of a rich local cluster, and that galaxy clusters began forming when the Universe was only ten per cent of its present age.

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