A VLT spectroscopic survey of RX J0152.7-1357, a forming cluster of galaxies at z=0.837

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 similar to 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 similar to 919 and similar to 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] (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.