The compact group--fossil group connection: observations of a massive compact group at z=0.22

Abstract

It has been suggested that fossil groups could be the cannibalized remains of compact groups, that lost energy through tidal friction. However, in the nearby universe, compact groups which are close to the merging phase and display a wealth of interacting features (such as HCG 31 and HCG 79) have very low velocity dispersions and poor neighborhoods, unlike the massive, cluster-like fossil groups studied to date. In fact, known z=0 compact groups are very seldom embedded in massive enough structures which may have resembled the intergalactic medium of fossil groups. In this paper we study the dynamical properties of CG6, a massive compact group at z=0.220 that has several properties in common with known fossil groups. We report on new g' and i' imaging and multi-slit spectroscopic performed with GMOS on Gemini South. The system has 20 members, within a radius of 1 h_70^-1 Mpc, a velocity dispersion of 700 km/s and has a mass of 1.8 x 10^14 h_70^-1 Msun, similar to that of the most massive fossil groups known. The merging of the four central galaxies in this group would form a galaxy with magnitude M_r' ~ -23.4, typical for first-ranked galaxies of fossil groups. Although nearby compact groups with similar properties to CG 6 are rare, we speculate that such systems occurred more frequently in the past and they may have been the precursors of fossil groups.