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 _ { 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 \sigma _ { 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 \sigma _ { V } = 300 –500 km s ^ { -1 } ) in the central South–West region and a high–velocity clump ( with \sigma _ { V } \sim 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 \sigma _ { 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 , 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 .