The most distant structure of galaxies known:: A protocluster at z=4.1

Imaging and spectroscopy with the Very Large Telescope have revealed 20 Lyalpha emitters within a projected distance of 1.3 Mpc and 600 km s(-1) of the luminous radio galaxy TN J1338-1942 at z = 4.1. Compared to the field density of Lyalpha emitters, this implies an overdensity on the order of 15. The structure has a projected size of at least 2.7 x 1.8 Mpc and a velocity dispersion of 325 km s(-1), which makes it the most distant structure known. Using the galaxy overdensity and assuming a bias parameter b = 3-5, the mass is estimated to be similar to10(15) M-.. The radio galaxy itself is surrounded by an uniquely asymmetric Lyalpha halo. Taken together with our previous data on PKS 1138-262 at z similar to 2.16, these results suggest that luminous radio sources are excellent tracers of high-density regions in the early universe, which evolve into present-day clusters. The statistics of bright radio sources and of concentrations in the Lyman break galaxy population are consistent with the picture that each of those concentrations harbors an active or passive luminous radio source.