Galaxy Aggregates in the Coma Cluster
  We present evidence for a new morphologically defined form of small-scale substructure in the Coma Cluster, which we call galaxy aggregates. These aggregates are dominated by a central galaxy, which is on average three magnitudes brighter than the smaller aggregate members nearly all of which lie to one side of the central galaxy. We have found three such galaxy aggregates dominated by the S0 galaxies RB 55, RB 60, and the star-bursting SBb, NGC 4858.   RB 55 and RB 60 are both equi-distant between the two dominate D galaxies NGC 4874 and NGC 4889, while NGC 4858 is located next to the larger E0 galaxy NGC 4860. All three central galaxies have redshifts consistant with Coma Cluster membership. We describe the spatial structures of these unique objects and suggest several possible mechanisms to explain their origin. These include: chance superpositions from background galaxies, interactions between other galaxies and with the cluster gravitational potential, and ram pressure. We conclude that the most probable scenario of creation is an interaction with the cluster through its potential. 
Christopher J. Conselice and John S. Gallagher III