Study of the Core of the Shapley Concentration: I.the Sample

We report the first results of a spectroscopic survey of galaxies in the core of the Shapley Concentration, the richest nearby supercluster of clusters of galaxies. We have measured 311 new galaxy redshifts in an area of $\sim 4.5$ square degrees centered around the Abell cluster A3558. Considering also the data already available in the literature, the total number of galaxy redshifts in this area amounts to more than 500. On the basis of these data we estimate the mean velocities and the velocity dispersions of the Abell clusters A3556, A3558 and the poor cluster SC 1329 -314. Finally, from an analysis of the projected and three--dimensional distributions of galaxies in this region, we estimate the galaxy overdensity and find that the core of the Shapley Concentration has an interesting, very complex dynamical state: the main clusters appear to be interacting with each other, forming a single elongated structure containing many subcondensations.