1999ApJ...521...90H

We present the ASCA spectral diagnostics of the X-ray-emitting gas of five clusters of galaxies in the Shapley concentration. X-rays from two rich clusters, A3562 and A3558, nicely follow the standard L_X-k_BT relation, while the X-ray luminosity of a poor cluster, A3556, comes below the relation line. The X-ray spectrum of another poor cluster, SC 1329-313, is found to be peculiar; the gas temperature is higher than that estimated from the velocity dispersion of its member galaxies. The energy of the iron Kα line is also higher than that expected from the temperature of the plasma considered in ionization equilibrium and hence is attributable to a plasma in the recombination-dominant phase. We infer that high-temperature gas and highly ionized iron are relics of past major merger events in the formation phase of poor clusters at several times 10⁸ yr ago. The putative merger heated the gas to high temperature by the associated shock and produced highly ionized iron as well. The high-temperature gas is gradually cooled down by adiabatic expansion, but the temperature is presently still higher than that expected from the velocity dispersion of galaxies. Iron atoms in the plasma, on the other hand, remain in highly ionized states with irregularities in the temperature distribution. We propose a dynamical sequence of clusters in the Shaply concentration; SC 1329-313 is an ongoing merger, A3556 is a candidate for postmerger, SC 1327-312 is a relaxed poor cluster, while A3562 and A3558 are already developed and rich clusters even with some mergers in succession. The Shapley concentration thus can be called a kindergarten of clusters of galaxies.