A New X-Ray–discovered Cluster of Galaxies Associated with CL 0016+16

We have discovered a new cluster of galaxies, RX J0018.3+1618, in a deep ROSAT PSPC pointing toward the distant cluster CL 0016+16. The mean radial velocities of the new cluster (z = 0.5506) and CL 0016+16 (z = 0.5455) differ by 1000 ± 390 km s-1, which is comparable to their internal velocity dispersions. The angular separation of the clusters, 95, corresponds to a projected linear distance of 4.2 Mpc. RX J0018.3+1618 has an X-ray luminosity in the 0.2-2 keV band of 1.3 × 1044 ergs s-1, which is about 10% of the luminosity of CL 0016+16. From the redshifts of seven galaxies in the new cluster, we derive a radial velocity dispersion of 540 -->+ 260−120 km s-1, which establishes RX J0018.3+1618 as one of the most distant known poor clusters. Simple energetic arguments suggest that the two clusters constitute a bound system. This discovery adds considerable weight to the suggestion made by Koo in 1981 that CL 0016+16 is part of a massive large-scale structure at z ~ 0.55.