RX J0848+4456: Disentangling a Moderate Redshift Cluster

We present a multiwavelength 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 Chandra X-Ray 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 a slightly lower redshift, z = 0.543, with little X-ray emission. This lower redshift system, however, is a gravitational lens, with the lensed galaxy being 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 × 1044 ergs 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 intracluster medium in equilibrium with the dark matter potential. The lower redshift group of galaxies at z = 0.543 has, at most, one-fifth, and 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 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.