A search for clusters at high redshift

VLT1 spectroscopic observations of Lyα-excess objects in the field of the clumpy radio galaxy 1138–262 at z = 2.16 have led to the discovery of 14 galaxies and one QSO at approximately the same distance as the radio galaxy. All galaxies have redshifts in the range 2.16±0.02, centered around the redshift of the radio galaxy, and are within a projected physical distance of 1.5 Mpc from it. The velocity distribution suggests that there are two galaxy subgroups having velocity dispersions of ∼ 500 km s−1 and ∼ 300 km s−1 and a relative velocity of 1800 km s−1. If these are virialized structures, the estimated dynamical masses for the subgroups are ∼9 and ∼3 × 1013 M respectively, implying a total mass for the structure of more than 1014 M . The new observations, together with previous results, suggest that the structure of galaxies around 1138–262 is likely to be a forming cluster.