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

Abstract

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 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, σv = 1322 -68+74 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 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 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. © ESO 2005.