Internal dynamics of the z ~ 0.8 cluster RX J0152.7-1357

We present the results from the dynamical analysis of the cluster of galaxies RXJ0152.7−1357, which shows a complex structure in its X-ray emission, with two major clumps in the central region and a third clump in the Eastern region. Our analysis is based on redshift data for 187 galaxies. We find that RXJ0152.7−1357 appears as a well isolated peak in the redshift space at z = 0.836, which includes 95 galaxies recognized as cluster members. We compute the line-of-sight velocity dispersion of galaxies, σV = 1322+74 −68 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 5-6 keV. We find evidence that this cluster is far from dynamical equilibrium, as shown by the non Gaussianity of the velocity distribution, the presence of a velocity gradient and a significant substructure. Our analysis shows that the high value of σV is due to the complex structure of RXJ0152.7−1357, i.e. to the presence of three galaxy clumps of different mean velocities. Using optical data we detect a low-velocity clump (with σV = 300–500 km s−1) in the central southwest region and a high-velocity clump (with σV ∼ 700 km s−1) in the Eastern region, corresponding well to the South–West and East peaks detected in the X-ray emission. The central North–East X-ray peak is associated to the main galaxy structure with a velocity which is intermediate between those of the other two clumps and σV ∼ 900 km s−1. The mass of the whole system within 2 Mpc is estimated to lie in the range (1.2−2.2) × 1015 M, depending on the model adopted to describe the cluster dynamics. Such values are comparable to those of very massive clusters at lower redshifts. Analytic calculations based on the two-body model indicate that the system is most likely bound and currently undergoing merging. In particular, we suggest that the southwestern clump is not a small group, but rather the dense cluster-core of a massive cluster, most likely destined to survive tidal disruption during the merger.