Internal dynamics of the radio halo cluster Abell 2744

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 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.

Flaming, bright galaxies along the filaments of A 2744

“…The ensuing mean redshift of the cluster (0.3068 ± 0.006) is in excellent agreement with previous determinations. The total number of 194 spectroscopic cluster members is twice as large as the number considered in the analysis of Boschin et al (2006). Furthermore, it probes an area almost four times as large.…”

VLA Radio Observations of the HST Frontier Fields Cluster Abell 2744: The Discovery of New Radio Relics

“…Kinematic studies also showed a bimodal velocity dispersion in the cluster center, with a third group of cluster members associated with the northwestern X-ray peak (Boschin et al 2006;Braglia et al 2009). …”

The Sloan great wall. Rich clusters

“…In Table 2 we give the values of kurtosis and skewness for the distributions of the peculiar velocities of galaxies in clusters (see also Solanes et al 1999;Oegerle & Hill 2001;Boschin et al 2006;Barrena et al 2007;Hwang & Lee 2007). The positive values of kurtosis in Table 2 suggest that all distributions are peaked.…”

“…An increasing number of studies have recently shown the presence of substructure in rich clusters (Solanes et al 1999;Oegerle & Hill 2001;Burgett et al 2004;Boschin et al 2006;Barrena et al 2007;Hwang & Lee 2007;Aguerri & Sanchez-Janssen 2010) and in poor clusters (Boschin et al 2008); Vennik & Hopp (2009) found a substructure also in poor groups. Tovmassian & Plionis (2009) studied the properties of poor groups from the SDSS survey and showed that many groups of galaxies presently are not in a dynamical equilibrium but at various stages of virialization.…”

“…Ashman et al 1994;Boschin et al 2006;Barrena et al 2007). In the R environment there is a suitable package to study these mixture models, called flexmix (Leisch 2004).…”