A 2163: Merger events in the hottest Abell galaxy cluster^*,**

I. Dynamical analysis from optical data

¹ Laboratoire Cassiopée, CNRS, UMR 6202, Observatoire de la Côte d'Azur, BP4229, 06304 Nice Cedex 4, France e-mail: sophie.maurogordato@oca.eu
² INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Institut für Astro- und Teilchenphysik, Innsbruck Universität, Technikerstrasse 25/8,6020 Innsbruck, Austria
⁴ Laboratoire d'Astrophysique de Toulouse-Tarbes, CNRS-UMR 5572 and Université Paul Sabatier Toulouse III, 14 Avenue Belin, 31400 Toulouse, France
⁵ Service d'Astrophysique, DAPNIA, CEA-CEN Saclay, 91191 Gif-sur-Yvette, France
⁶ Max-Planck-Institut für extraterrestriche Physik, Giessenbachstrasse, 85748 Garching, Germany
⁷ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy

Received: 5 April 2007
Accepted: 6 November 2007

Abstract

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 , the galaxy distribution shows a clear elongation approximately with the east-west axis extending over  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 , 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 and  km s^-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 ~59 200 km s^-1 and 60 500 km s^-1. A significant velocity gradient (~1250 km s^-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 .


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 A 2163-A has undergone a recent ( Gyr) merger along a NE/SW (or E-W) axis, and A 2163-B is connected to the main complex, and is probably infalling on A 2163-A.