A2163: Merger events in the hottest Abell galaxy cluster I. Dynamical analysis from optical data

Context. A2163 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 A2163 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(70)(-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 +/- 0.0003 and 1434 +/- 60 km s(-1). We spectroscopically confirm that the northern and western components (A2163-B and A2163-C) belong to the A2163 complex. The velocity distribution shows multi-modality, with an overall bimodal structure peaking at similar to 59 200 km s(-1) and similar to 60 500 km s(-1). A significant velocity gradient (similar to 1250 km s(-1)) is detected along the NE/SW axis of the cluster, which partially explains the detected bimodality. A2163 appears to be exceptionally massive: the cluster virial mass is M-vir = 3.8 +/- 0.4 x 1015 M-circle dot h(70)(-1). Conclusions. Our analysis of the optical data, combined with the available information from X-ray observations and predictions of numerical simulations, supports a scenario in which A2163-A has undergone a recent (t similar to 0.5 Gyr) merger along a NE/SW (or E-W) axis, and A2163-B is connected to the main complex, and is probably infalling on A2163-A.