Internal dynamics of the z ∼ 0 . 8 cluster RX J 0152 . 7 − 1357 ⋆

We present the results from the dynamical analysis of the cluster of galaxies RX J0152.7−1357, 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 RX J0152.7−1357 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, σ V = 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 significant substructure. Our analysis shows that the high value of σ V is due to the complex structure of RX J0152.7−1357, i.e. to the presence of three galaxy clumps of different mean velocity. Using optical data we detect a low–velocity clump (with σ V = 300–500 km s −1) in the central South– West region and a high–velocity clump (with σ V ∼ 700 km s −1) in the Eastern region, well corresponding 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 σ V ∼ 900 km s −1. The mass of the whole system within 2 Mpc is estimated to lie in the range (1.2-2.2)×10 15 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, currently undergoing merging. In particular, we suggest that the South–West clump is not a small group, but rather the dense cluster–core of a massive cluster, likely destined to survive tidal disruption during the merger.