The build-up of the Coma cluster by infalling substructures

¹ Laboratoire d'Astropysique de Marseille, UMR 6110 CNRS-Université de Provence, Traverse du Siphon, Les Trois Lucs, 13012 Marseille, France e-mail: christophe.adami@oamp.fr
² INAF – Osservatorio Astronomico di Trieste, via GB Tiepolo 11, 34131, Trieste, Italy
³ Institut d'Astrophysique de Paris, UMR 7095, Université Pierre & Marie Curie, 98bis Bd. Arago, 75014 Paris, France
⁴ Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France

Accepted: 24 May 2005

Abstract

We present a new multiwavelength analysis of the Coma cluster subclustering based on recent X-ray data and on a compilation of nearly 900 redshifts. We characterize subclustering using the Serna & Gerbal (1996, A&A, 309, 65) hierarchical method, which makes use of galaxy positions, redshifts, and magnitudes, and identify 17 groups. One of these groups corresponds to the main cluster, one is the well known group associated with the infalling galaxy NGC 4839, and one is associated with NGC 4911/NGC 4926. About one third of the 17 groups have velocity distributions centered on the velocities of the very bright cluster galaxies they contain (magnitudes ).
In order to search for additional substructures, we made use of the isophotes of X-ray brightness residuals left after the subtraction of the best-fit β-model from the overall X-ray gas distribution (Neumann et al. 2003, A&A, 400, 811). We selected galaxies within each of these isophotes and compared their velocity distributions with that of the whole cluster. We confirm in this way the two groups associated, respectively, with NGC 4839, and with the southern part of the extended western substructure visible in X-rays.
We discuss the group properties in the context of a scenario in which Coma is built by the accretion of groups infalling from the surrounding large-scale structure. We estimate the recent mass accretion rate of Coma and compare it with hierarchical models of cluster evolution.