Abell548: A rich cluster of galaxies in the making

We performed an in-depth study of the complex structure of the galaxy cluster A548, based on 108 spectroscopic galaxy velocities reported in this paper. This analysis includes 80 new redshifts, along with velocity data from the literature and a reanalysis of X-ray observations. We find that the cluster is in a merging phase of at least six components. These belong to two sheets, or clouds, of galaxies with a mean velocity difference of 1100–1300 km s −1 . Each cloud has broken into three concentrations, with a relative separation between their centres of approximately ∼1° on the plane of the sky and velocity separations of nearly 1000 km s −1 , all visible in the X-ray maps. The western component (CID36) is primarily composed of a single, small concentration, the middle component (A548W) has two principal substructures, with the largest being in an advanced merging state, involving two secondary components centred on a dominant dumbbell galaxy. The far-eastern component (A548E) is the merger of two corresponding substructures. There is a further extension to the NE, which is not observed in X-rays. This extension is divided into two sub-clumps with velocities corresponding to those in the clouds and located in front of A3367. Consequently, the two clouds have a large coherence length along the plane of the sky. The entire region can be considered as the ongoing phases of a rich cluster formation, which will have a central dumbbell as its central galaxy.