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

¹ ESO-European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching b. München, Germany
² Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Str., Baltimore, MD 21218, USA e-mail: demarco@pha.jhu.edu
³ ESO-European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
⁴ Kavli Institute for Theoretical Physics, Kohn Hall, University of California, Santa Barbara, CA 93106, USA
⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching b. München, Germany
⁶ INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
⁷ Dipartimento di Astronomia, Università degli Studi di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁸ Department of Physics, University of California, Davis, 1 Shields Avenue, Davis, CA 95616, USA
⁹ Participating Guest, Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, USA
¹⁰ Dipartimento di Astronomia dell'Università di Trieste, via Tiepolo 11, 34131 Trieste, Italy
¹¹ INFN - National Institute for Nuclear Physics, Trieste Università, via Valerio 2, 34127 Trieste, Italy
¹² Astrophysics Department, University of Oxford, Keble Road, Oxford OX1 3RH
¹³ SIRTF Science Center, California Institute of Technology, Pasadena, CA 91125, USA
¹⁴ Racah Institute of Physics, Hebrew University, Jerusalem 91904, Israel

Received: 31 August 2004
Accepted: 2 November 2004

Abstract

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  ± 0.001 and we estimate the velocity dispersion of the overall cluster galaxy distribution to be ~. 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 ~ 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.