Internal dynamics of the radio halo cluster Abell 2744

Free access article

Issue		A&A Volume 449, Number 2, April II 2006


Page(s)		461 - 474
Section		Cosmology (including clusters of galaxies)
DOI		http://dx.doi.org/10.1051/0004-6361:20054408

A&A 449, 461-474 (2006)
DOI: 10.1051/0004-6361:20054408

Internal dynamics of the radio halo cluster Abell 2744

W. Boschin^{1, 2}, M. Girardi^{2, 3}, M. Spolaor² and R. Barrena⁴

¹ Fundación Galileo Galilei - INAF, C/Alvarez de Abreu 70, 38700 Santa Cruz de La Palma, Canary Islands, Spain
e-mail: boschin@tng.iac.es
² Dipartimento di Astronomia, Università degli Studi di Trieste, via Tiepolo 11, 34131 Trieste, Italy
³ INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁴ Instituto de Astrofisica de Canarias, C/Via Lactea s/n, 38200 La Laguna, Tenerife, Canary Islands, Spain

(Received 25 October 2005 / Accepted 15 November 2005)

Abstract
Aims.We present a detailed dynamical analysis of the rich galaxy cluster A2744, containing a powerful diffuse radio halo.
Methods.Our analysis is based on redshift data for 102 galaxies, part of them recovered from unexplored spectra in the ESO archive. We combine galaxy velocity and position information to select the cluster members and determine global dynamical properties of the cluster. We use a variety of statistical tests to detect possible substructures.
Results.We find that A2744 appears as a well isolated peak in the redshift space at $\left<z\right>=0.306$ , which includes 85 galaxies recognized as cluster members. We compute the line-of-sight (LOS) velocity dispersion of galaxies, $\sigma_{\rm V}=1767_{-99}^{+121}$ km s^-1, which is significantly larger than what is expected in the case of a relaxed cluster with an observed X-ray temperature of 8 keV. We find evidence that this cluster is far from dynamical equilibrium, as shown by the non-Gaussian nature of the velocity distribution, the presence of a velocity gradient and a significant substructure. Our analysis shows the presence of two galaxy clumps of different mean LOS velocities $\Delta V \sim 4000$ km s^-1. We detect a main, low-velocity clump with $\sigma_{\rm V}\sim 1200{-}1300$ km s $^{-1} \;$ and a secondary, high-velocity clump with $\sigma_{\rm V}= 500{-}800$ km s^-1and located in the S-SW cluster region. We estimate a cluster mass within 1 Mpc $\;$ of $1.4{-}2.4\times 10^{15}~M_{\odot}$ , depending on the model adopted to describe the cluster dynamics.
Conclusions.Our results suggest a merging scenario of two clumps with a mass ratio of 3:1 and a LOS impact velocity of $\Delta V_{\rm rf} \sim 3000$ km s^-1, likely observed just after the core passage. The merging is occuring roughly in the NS direction with the axis close to the LOS. This scenario agrees with that proposed on the basis of recent Chandra results in its general lines although suggesting a somewhat more advanced merging phase. Our conclusions support the view of the connection between extended radio emission and energetic merging phenomena in galaxy clusters.

Key words: galaxies: clusters: general -- galaxies: clusters: individual: Abell 2744 -- galaxies: distances and redshifts -- intergalactic medium -- cosmology: observations

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