Cl 1205+44, a fossil group at z = 0.59
  This is a report of Chandra, XMM-Newton, HST and ARC observations of an extended X-ray source at z = 0.59. The apparent member galaxies range from spiral to elliptical and are all relatively red (i'-Ks about 3). We interpret this object to be a fossil group based on the difference between the brightness of the first and second brightest cluster members in the i'-band, and because the rest-frame bolometric X-ray luminosity is about 9.2x10^43 h70^-2 erg s^-1. This makes Cl 1205+44 the highest redshift fossil group yet reported. The system also contains a central double-lobed radio galaxy which appears to be growing via the accretion of smaller galaxies. We discuss the formation and evolution of fossil groups in light of the high redshift of Cl 1205+44. 
M.P. Ulmer, C. Adami, G. Covone, F. Durret, G.B. Lima Neto, K.   Sabirli, B. Holden, R.G. Kron, A.K. Romer