Another X-ray-discovered poor cluster of galaxies associated with CL 0016 + 16

We report optical spectroscopic observations of RX J0018.8 + 1602, a ROSAT X-ray source proposed to be an intermediate-redshift cluster of galaxies. Our observations confirm the identification of RX J0018.8 + 1602 and provide measurements of its mean radial velocity (z = 0.5406 ± 0.0006) and velocity dispersion (σ_p = 200_-80⁺¹¹⁰ km s^-1). This is the second poor cluster that has been found to be a companion to CL 0016 + 16 (z = 0.5455), the other one being RX J0018.3 + 1618 (z = 0.5506). The 0.2-2 keV band source-frame X-ray luminosity summed over both companion clusters is 5 × 10⁴⁴ ergs s^-1, which is a significant fraction (∼23%) of the X-ray luminosity of the main cluster. The companions are located at angular distances of 10′-25′ (minimum physical scales of 5-12 Mpc) from CL 0016 + 16, and we propose that they represent a new large-scale component of the X-ray emission from clusters of galaxies. Similar low X-ray luminosity poor clusters surrounding nearby Abell clusters can explain the excess power observed in the angular cross-correlation function between Abell clusters and the X-ray back-ground on inferred physical scales of 14-20 Mpc.