  Imaging and spectroscopy with the Very Large Telescope have revealed 20
Lyman-alpha emitters within a projected distance of 1.3 Mpc and 600 km/s of the
luminous radio galaxy TN J1338-1942 at z=4.1. Compared to the field density of
Lyman-alpha emitters, this implies an overdensity on the order of 15. The
structure has a projected size of at least 2.7 Mpc x 1.8 Mpc and a velocity
dispersion of 325 km/s, 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 ~10^15 M_sun. The radio galaxy itself is surrounded by an
uniquely asymmetric Lyman-alpha halo. Taken together with our previous data on
PKS 1138-262 at z ~ 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 harbours an active or passive
luminous radio source.
