An XMM-Newton observation of A3921: An off-axis merger^*

¹ Service d'Astrophysique, CEA Saclay, L'Orme des Merisiers, Bât. 709, 91191 Gif-sur-Yvette Cedex, France e-mail: jsauvageot@cea.fr
² H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK e-mail: e.belsole@bristol.ac.uk
³ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany e-mail: gwp@mpe.mpg.de
⁴ Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France
⁵ Dipartimento di Fisica, Universitá degli Studi di Roma “Tor Vergata”, via della Ricerca Scientifica, 1, 00133 Roma, Italy e-mail: Herve.Bourdin@roma2.infn.it

Received: 12 May 2004
Accepted: 22 September 2004

Abstract

We present the results of a detailed analysis of the XMM-Newton  observation of the galaxy cluster Abell 3921. The X-ray morphology of the cluster is elliptical, with the centroid offset from the brightest cluster galaxy by 17´´, and with a pronounced extension toward the NW. Subtraction of a 2D β-model fit to the main cluster emission reveals a large scale, irregular residual structure in the direction of the extension, containing both diffuse emission from the intra cluster medium, and extended emission from the second and third-brightest cluster galaxies (BG2 and BG3). The greatest concentration of galaxies in the subcluster lies at the extreme northern edge of the residual. The cluster exhibits a remarkable temperature structure, in particular a bar of significantly hotter gas, oriented SE-NW and stretching from the centre of the cluster towards BG2 and BG3. Our detailed study of the morphological and thermal structure points to an off-axis merger between a main cluster and a less massive galaxy cluster infalling from the SE. From comparison of the temperature map with numerical simulations, and with independent calculations based on simple physical assumptions, we conclude that the merging event is ∼0.5 Gyr old. The cluster is thus perhaps the best X-ray observed candidate so far of an intermediate mass ratio, moderate impact parameter merger.