ar X iv : a st ro - p h / 01 05 01 1 v 1 1 M ay 2 00 1 RX J 0848 + 4456 : Disentangling a Moderate Redshift Cluster

We present a multi-wavelength study of RX J0848+4456, a cluster of galaxies discovered through X-ray emission in the ROSAT Deep Cluster Survey. Our observations consist of WFPC2 imaging, optical spectra, and X-ray data collected with the Chan-dra observatory. We find that RX J0848+4456 consists of an X-ray emitting cluster of galaxies at a redshift of z = 0.570 and a group at slightly lower redshift, z = 0.543, with little X-ray emission. This lower redshift system, however, is a gravitational lens, with the lensed galaxy an unusual AGN or star-forming system at z = 3.356. The cluster has an X-ray temperature of kT = 3.6±0.4 keV, a bolometric luminosity of 1.0 ± 0.3 × 10 44 erg s −1 and a velocity dispersion of 670 ± 50 km s −1. These values all agree with the low redshift correlations for clusters of galaxies, implying a relaxed system with the ICM in equilibrium with the dark matter potential. The lower redshift group of galaxies at z = 0.543 has, at most 1 5 more likely 1 10 , of the X-ray luminosity of RX J0848+4456. Despite being a gravitational lens, this is a low mass system, with an X-ray temperature of kT = 2.3 +0.5 −0.4 keV and a velocity dispersion of only 430±20 km s −1. Our observations show the importance of detailed studies of clusters of galaxies when using them as probes of cosmological mass functions.