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 \pm 0.0006 ) and velocity dispersion ( \sigma _ { p } = 200 ^ { +110 } _ { -80 } 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 \times 10 ^ { 44 } ergs s ^ { -1 } , which is a significant fraction \sim 23 % of the X-ray luminosity of the main cluster .
The companions are located at angular distances of 10 ^ { \prime } to 25 ^ { \prime } ( 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 clusters can explain the excess power observed in the angular cross-correlation function between Abell clusters and the X-ray background on inferred physical scales of 14–20 Mpc ( Soltan et al .
1996 ) .