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 one square degree ($\sim$ 3 Mpc$^2$) centeredon the cluster cD galaxy, we have obtained a statistically complete photometriccatalog with positions and magnitudes of 1421 galaxies (down to a limitingmagnitude of $B \sim 21$). This catalog has been matched to the recent velocitydata obtained by Mazure et al. (1997) and from the literature, yielding aradial velocity catalog containing 322 galaxies. Our analysis shows that theposition/velocity space distribution of galaxies shows significantsubstructure. A central bimodal core detected previously in preliminary studiesis confirmed by using the Adaptive Kernel Technique and Wavelet Analysis. Weshow that this central bimodal subtructure is nevertheless composed of aprojected feature, kinematically unrelated to the cluster, plus a group ofgalaxies probably in its initial merging phase into a relaxed core. The cDvelocity offset with respect to the average cluster redshift, reported earlierby several authors, is completely eliminated as a result of our dynamicalanalysis. The untangling of the relaxed core component also allows a better,more reliable determination of the central velocity dispersion, which in turneliminates the ``$\beta$-problem'' for A3558. The cluster also shows a``preferential'' distribution of subclumps coinciding with the direction of themajor axis position angle of the cD galaxy and of the central X-ray emissionellipsoidal distribution, in agreement with an anisotropic merger scenario.Comment: 35 pages in latex, 17 figures in Postscript, accepted for publication in the Astrophysical Journa