Spectroscopy of clusters in the ESO Distant Cluster Survey (EDisCS) ^*,**

Redshifts, velocity dispersions and substructure for 5 clusters

¹ Osservatorio Astronomico, vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: [halliday;poggianti]@pd.astro.it
² Max-Planck-Institut für Astrophysik, Karl-Schwarschild-Str. 1, Postfach 1317, 85741 Garching, Germany e-mail: [halliday;lucia;grudnick;swhite]@mpa-garching.mpg.de
³ Max-Planck Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany e-mail: [milvang;saglia]@mpe.mpg.de
⁴ GEPI, CNRS-UMR8111, Observatoire de Paris, section de Meudon, 5 Place Jules Janssen, 92195 Meudon Cedex, France e-mail: Pascale.Jablonka@obspm.fr
⁵ School of Physics and Astronomy, University of Nottingham, University Park, NG7 2RD, UK e-mail: Alfonso.Aragon@nottingham.ac.uk
⁶ Laboratoire d'Astrophysique, UMR 5572, Observatoire Midi-Pyrenees, 14 Avenue E. Belin, 31400 Toulouse, France e-mail: Roser.Pello@ast.obs-mip.fr
⁷ Herzberg Institute of Astrophysics, National Research Council of Canada, Victoria, BC V9E 2E7, Canada e-mail: Luc.Simard@nrc.ca
⁸ Institut für Astrophysik und Extraterrestrische Forschung, Universität Bonn, Auf dem Hugel 71, 53121 Bonn, Germany
⁹ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA e-mail: [clowe;dzaritsky]@as.arizona.edu
¹⁰ Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195, USA e-mail: jd@astro.washington.edu

Received: 17 May 2004
Accepted: 28 July 2004

Abstract

We present spectroscopic observations of galaxies in 4 clusters at and in one cluster at obtained with the FORS2 spectrograph on the VLT as part of the ESO Distant Cluster Survey (EDisCS), a photometric and spectroscopic survey of 20 intermediate to high redshift clusters. We describe our target selection, mask design, observation and data reduction procedures, using these first 5 clusters to demonstrate how our strategies maximise the number of cluster members for which we obtain spectroscopy. We present catalogues containing positions, I-band magnitudes and spectroscopic redshifts for galaxies in the fields of our 5 clusters. These contain 236 cluster members, with the number of members per cluster ranging from 30 to 67. Our spectroscopic success rate, i.e. the fraction of spectroscopic targets which are cluster members, averages 50% and ranges from 30% to 75%. We use a robust biweight estimator to measure cluster velocity dispersions from our spectroscopic redshift samples. We also make a first assessment of substructure within our clusters. The velocity dispersions range from 400 to 1100 . Some of the redshift distributions are significantly non-Gaussian and we find evidence for significant substructure in two clusters, one at and the other at . Both have velocity dispersions exceeding 1000  but are clearly not fully virialised; their velocity dispersions may thus be a poor indicator of their masses. The properties of these first 5 EDisCS clusters span a wide range in redshift, velocity dispersion, richness and substructure, but are representative of the sample as a whole. Spectroscopy for the full dataset will allow a comprehensive study of galaxy evolution as a function of cluster environment and redshift.