Internal dynamics of the radio halo cluster A 2744

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 z = 0.306, which includes 85 galaxies recognized as cluster members. We compute the line–of–sight (LOS) velocity dispersion of galaxies, σ V = 1767 +121 −99 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 ∆V ∼ 4000 km s −1. We detect a main, low–velocity clump with σ V ∼ 1200-1300 km s −1 and a secondary, high–velocity clump with σ V = 500-800 km s −1 and located in the S–SW cluster region. We estimate a cluster mass within 1 Mpc of 1.4–2.4 ×10 15 M ⊙ , 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 ∆V rf ∼ 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.