6 v 1 1 0 A pr 1 99 7 A MASSIVE CLUSTER OF GALAXIES AT z = 0 . 996

We report the identification of a cluster of galaxies around the high-redshift radio galaxy 3CR184 at z = 0.996. The identification is supported by an excess of galaxies observed in projection in I band images (both in ground-based and HST data), a peak in the redshift distribution comprising 11 galaxies (out of 56 with measured redshifts) in a ∼ 2000 km s velocity interval, and the observation on HST WFPC2 frames of a gravitational arc seen projected at 42h 50 kpc away from the central radio galaxy. We thus have strong evidence for the presence of a massive cluster at z ≃ 1. The mass contained within the arc radius is in the range [1.20× 1013h 50 M⊙, 2.78× 10 13h 50 M⊙] for zarc within the interval 3-1.5; the corresponding mass to light ratio varies from 56h50 to 140h50. The velocity dispersion deduced from the galaxy cluster redshifts is 634 −102km s, leading to a virial mass M = 6.16 −2.40 × 10 14h 50 M⊙ and a mass to light ratio 200h50 < (M/LB)400h−1 50 < 500h50 within a radius of 400h −1 50 kpc.