A Massive Cluster of Galaxies at Z = 0.996

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_solar for z_arc 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_solar and a mass-to-light ratio of 200 h₅₀ < (M/L_B)_400h^-150 < 500 h₅₀ within a radius of 400 h^-1₅₀ kpc.

Partly based on observations obtained with the NASA-ESA Hubble Space Telescope.