A SUBSTRUCTURE-ANALYSIS OF THE A3558 CLUSTER COMPLEX

The 'algorithm driven by the density estimate for the identification o f clusters' (DEDICA) is applied to the A3558 cluster complex in order to find substructures. This complex, located at the centre of the Shap ley Concentration supercluster, is a chain formed by the ACO clusters A3556, A3558 and A3562 and the two poor clusters SC 1327-312 and SC 13 29-313. We find a large number of clumps, indicating that strong dynam ical processes are active. In particular, it is necessary to use a ful ly three-dimensional sample (i.e. using the galaxy velocity as third c oordinate) in order also to recover the clumps superimposed along the line of sight. Even though a large number of detected substructures wa s already found in a previous analysis, this method is more efficient and faster when compared with a wide battery of tests, and permits the direct estimate of the detection significance. Almost all subclusters previously detected by the wavelet analyses found in the literature a re recognized by DEDICA. On the basis of the substructure analysis, we also briefly discuss the origin of the A3558 complex by comparing two hypotheses: (i) the structure is a cluster-cluster collision seen jus t after the first core-core encounter; or (ii) this complex is the res ult of a series of incoherent group-group and cluster-group mergings, focused in that region by the presence of the surrounding supercluster . We studied the fraction of blue galaxies in the detected substructur es and found that the bluest groups reside between A3562 and A3558, i. e. in the expected position for the scenario of cluster-cluster collis ion.