A Massive Cluster of Galaxies at [CLC][ITAL]z[/ITAL][/CLC] = 0.996

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Abstract
We report the identification of a cluster of galaxies around the high-redshift radio galaxy 3CR 184 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 Hubble Space Telescope [HST] data), a peak in the redshift distribution comprising 11 galaxies (out of 56 with measured redshifts) in a ~2000 km s-1 velocity interval, and the observation on HST WFPC2 frames of a gravitational arc seen projected at 42 h−150 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 (1.20-2.78) × 1013 h−150 M☉ for zarc between 1.5 and 3; the corresponding mass-to-light ratio varies from 56 h50 to 140 h50. The velocity dispersion deduced from the galaxy cluster redshifts is 634+ 206−102 km s-1, leading to a virial mass M = 6.16+ 3.94−2.40 × 1014 h−150 M☉ and a mass-to-light ratio of 200 h50 < (M/LB)400 h−150 < 500 h50 within a radius of 400 h−150 kpc.
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