2008A&A...481..593M

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. 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. 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. 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 h₇₀^–1Mpc, 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±60km/s. 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 ∼59200km/s and ∼60500km/s. A significant velocity gradient (∼1250km/s) 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±0.4x10¹⁵M_☉h₇₀^–1. 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∼0.5Gyr) 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.