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 ± 0.001 and we estimate the velocity dispersion of the overall cluster
galaxy distribution to be ∼1600 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 ∼919 and ∼737 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] (λ3727) 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.
