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 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, $\sigma_{\rm V}=1322^{+74}_{-68}$ 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 a significant substructure. Our analysis shows that the high value of $\sigma_{\rm V}$ is due to the complex structure of RX J0152.7-1357, i.e. to the presence of three galaxy clumps of different mean velocities. Using optical data we detect a low-velocity clump (with $\sigma_{\rm V}=300$–500 km s -1 ) in the central southwest region and a high-velocity clump (with $\sigma_{\rm V} \sim 700$ 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 $\sigma_{\rm V}\sim 900$ km s -1 . The mass of the whole system within 2 Mpc is estimated to lie in the range $(1.2{-}2.2)\times 10^{15}$ $M_{\odot}$, 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.