Cl 1205+44: A Fossil Group at z = 0.59

This is a report of Chandra, XMM-Newton, Hubble Space Telescope, and Astrophysical Research Consortium telescope 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^'-K_s 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 9.2×10⁴³h^-2₇₀ ergs s^-1. This makes Cl 1205+44 the highest redshift fossil group yet reported. The system also contains a central double-lobed radio galaxy that 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.