Internal dynamics of the radio halo cluster Abell 2744
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
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 
, which includes 85 galaxies recognized as
cluster members. We compute the line-of-sight (LOS) velocity
dispersion of galaxies, 
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
km s-1. We detect a main, low-velocity clump with
km s
and a secondary, high-velocity
clump with 
km s-1and located in the S-SW
cluster region. We estimate a cluster mass within 1 Mpc
of
, 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 
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.
© ESO, 2006