EVIDENCE OF SUBSTRUCTURE IN THE CLUSTER OF GALAXIES A3558

We investigate the dynamical properties of the cluster of galaxies A35 58 (Shapley 8). Studying a region of 1 deg(2) (similar to 3 Mpc(2)) ce ntered on the cluster cD galaxy, we have obtained a statistically comp lete photometric catalog with positions and magnitudes of 1421 galaxie s (down to a limiting magnitude of B similar to 21). This catalog has been matched to the recent velocity data obtained by Mazure et al. and from the literature, yielding a radial velocity catalog containing 32 2 galaxies. We analyze the resulting catalog in search of substructure , using different statistical techniques. This analysis shows that the position/velocity space distribution of galaxies shows significant su bstructure. A central bimodal core, detected previously in a prelimina ry study by Dantas, de Carvalho, & Capelato and by an analysis of the X-ray emission map by Slezak, Durret, & Gerbal, is confirmed using the adaptive kernel technique and wavelet analysis. We show that this cen tral bimodal substructure is nevertheless composed of a projected feat ure, kinematically unrelated to the cluster, plus a group of galaxies probably in its initial merging phase into a relaxed core. The cD gala xy's velocity offset with respect to the average cluster redshift, rep orted earlier by several authors, is completely eliminated as a result of our dynamical analysis. The untangling of the relaxed core compone nt also allows a better, more reliable determination of the central ve locity dispersion, which in turn eliminates the ''beta problem'' for A 3558. The cluster also shows a ''preferential'' distribution of subclu mps coinciding with the direction of the major-axis position angle of the cD galaxy and of the central X-ray emission ellipsoidal distributi on, in agreement with the anisotropic merger scenario described by Wes t.