Evidence of substructure in the cluster of galaxies A3558

We investigate the dynamical properties of the cluster of galaxies A3558 (Shapley 8). Studying a region of 1 deg(2) (similar to 3 Mpc(2)) centered on the cluster cD galaxy, we have obtained a statistically complete photometric catalog with positions and magnitudes of 1421 galaxies (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 322 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 substructure. A central bimodal core, detected previously in a preliminary 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 central bimodal substructure is nevertheless composed of a projected feature, kinematically unrelated to the cluster, plus a group of galaxies probably in its initial merging phase into a relaxed core. The cD galaxy's velocity offset with respect to the average cluster redshift, reported earlier by several authors, is completely eliminated as a result of our dynamical analysis. The untangling of the relaxed core component also allows a better, more reliable determination of the central velocity dispersion, which in turn eliminates the ''beta problem'' for A3558. The cluster also shows a ''preferential'' distribution of subclumps coinciding with the direction of the major-axis position angle of the cD galaxy and of the central X-ray emission ellipsoidal distribution, in agreement with the anisotropic merger scenario described by West.