Another X - ray – Discovered Poor Cluster of Galaxies Associated with CL 0016 + 16 1

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 (σp = 200 +110 −80 km s ). 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× 10 ergs s, which is a significant fraction ∼23% of the X-ray luminosity of the main cluster. The companions are located at angular distances of 10 to 25 (minimum physical scales of 5 Mpc to 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 Optical observations reported here were obtained at the Multiple Mirror Telescope, a joint facility of the Smithsonian Institution and the University of Arizona.