2005A&A...442...29G

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 RXJ0152.7-1357appears 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_–68km/s, which is significantly larger than what is expected in the case of a relaxed cluster with an observed X-ray temperature of 5-6keV. 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 σ_V 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 σ_V=300-500km/s) in the central southwest region and a high-velocity clump (with σ_V∼700km/s) 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 σ_V∼900km/s. The mass of the whole system within 2Mpc is estimated to lie in the range (1.2-2.2)x10¹⁵M_☉, 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.