A multi-wavelength study of the merger candidate Abell S0721 and the activities of its member galaxies

We present a multi-wavelength study of the merging history and its influence on galaxy activities of a newly detected merger candidate - Abell S0721. This cluster was only known as a poor cluster in the Shapley Supercluster, but when several survey data of different wavelengths are combined, evidence of strong merging is revealed. We have analyzed the optical and X-ray structures of the cluster, as well as galaxy dynamics and properties of the radio galaxies in the cluster. The dynamics analysis shows the velocity distribution of this system is significantly deviated from a 1-Gaussian model, indicating the existence of 2-3 dynamically different sub-systems; in addition, both optical and X-ray structures of the cluster show an elongated and multi-clump structure, two X-ray clumps imaged by ROSAT All Sky Survey are well correlated with two galaxy density peaks. This cluster is possibly a merger of several groups; our analysis shows that the projection angle for two clumps in the main structure is possibly near the projection plane, and the substructure is seen at ∼30-75° from the projection plane. The merging process could also be the origin to boost the activities of its member galaxies. Finally, as compared with other merger candidates in the Shapley Supercluster, the radio galaxies identified in Abell S0721 are all optically luminous, and the fraction of luminous radio galaxies is only lower than one merger candidate (which is proposed to be at the very beginning of a merger event), this suggests Abell S0721 is also at the beginning of a merger event.