We present the results of an extensive spectroscopic survey of RX~J0152.7-1357 , one of the most massive distant clusters of galaxies known .
Multi-object spectroscopy , carried out with FORS1 and FORS2 on the ESO Very Large Telescope ( VLT ) , has allowed us to measure more than 200 redshifts in the cluster field and to confirm 102 galaxies as cluster members .
The mean redshift of the cluster is z = 0.837 \pm 0.001 and we estimate the velocity dispersion of the overall cluster galaxy distribution to be \sim 1600 \mathrm { km s ^ { -1 } } .
The distribution of cluster members is clearly irregular , with two main clumps that follow the X-ray cluster emission mapped by Chandra .
A third clump of galaxies to the east of the central structure and at the cluster redshift has also been identified .
The two main clumps have velocity dispersions of \sim 919 and \sim 737 \mathrm { km s ^ { -1 } } respectively , and the peculiar velocity of the two clumps suggests that they will merge into a single more massive cluster .
A segregation in the star formation activity of the member galaxies is observed .
All star forming galaxies are located outside the high-density peaks , which are populated only by passive galaxies .
A population of red galaxies ( belonging to the cluster red sequence ) with clear post-starburst spectral features and [ OII ] ( \lambda 3727 ) emission lines is observed in the outskirts of the cluster .
Two AGNs , which were previously confused with the diffuse X-ray emission from the intracluster medium in ROSAT and BeppoSAX observations , are found to be cluster members .