  The most massive galaxies and the richest clusters are believed to have
emerged from regions with the largest enhancements of mass density 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-alpha line of hydrogen.
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 Lya, due to absorption of radiation from the galaxy by the
intervening galactic medium. Here we report multicolour imaging of the most
distant candidate 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 10 per cent of its present
age.
