A 2163 : Merger events in the hottest Abell galaxy cluster I. Dynamical analysis from optical data

Context. A 2163 is among the richest and most distant Abell clusters, presenting outstanding properties in different wavelength domains. X-ray observations have revealed a distorted gas morphology and strong features have been detected in the temperature map, suggesting that merging processes are important in this cluster. However, the merging scenario is not yet well-defined. Aims. We have undertaken a complementary optical analysis, aiming to understand the dynamics of the system, to constrain the merging scenario and to test its effect on the properties of galaxies. Methods. We present a detailed optical analysis of A 2163 based on new multicolor wide-field imaging and medium-to-high resolution spectroscopy of several hundred galaxies. Results. The projected galaxy density distribution shows strong subclustering with two dominant structures: a main central component (A), and a northern component (B), visible both in optical and in X-ray, with two other substructures detected at high significance in the optical. At magnitudes fainter than R =19, the galaxy distribution shows a clear elongation approximately with the east-west axis extending over 4 h sub(70) super(-1) Mpc, while a nearly perpendicular bridge of galaxies along the north-south axis appears to connect (B) to (A). The (A) component shows a bimodal morphology, and the positions of its two density peaks depend on galaxy luminosity: at magnitudes fainter than R = 19, the axis joining the peaks shows a counterclockwise rotation (from NE/SW to E-W) centered on the position of the X-ray maximum. Our final spectroscopic catalog of 512 objects includes 476 new galaxy redshifts. We have identified 361 galaxies as cluster members; among them, 326 have high precision redshift measurements, which allow us to perform a detailed dynamical analysis of unprecedented accuracy. The cluster mean redshift and velocity dispersion are respectively z= 0.2005 \pm 0.0003 and 1434 \pm 60 km s super(-1). We spectroscopically confirm that the northern and western components (A 2163-B and A 2163-C) belong to the A 2163 complex. The velocity distribution shows multi-modality, with an overall bimodal structure peaking at similar to 59 200 km s super(-1) and {\sim}60 500 km s super(-1). A significant velocity gradient ( similar to 1250 km s super(- 1)) is detected along the NE/SW axis of the cluster, which partially explains the detected bimodality. A 2163 appears to be exceptionally massive: the cluster virial mass is M_{\