Multi-object spectroscopy of low redshift EIS clusters

III. Properties of optically selected clusters

¹ Copenhagen University Observatory, Juliane Maries Vej 30, 2100 Copenhagen, Denmark e-mail: lisbeth@astro.ku.dk
² Observatoire de la Côte d'Azur, Laboratoire Cassiopée, BP 4229, 06304 Nice Cedex 4, France
³ European Southern Observatory, Karl-Schwartzschild-Str. 2, 85748 Garching b. München, Germany

Received: 16 August 2004
Accepted: 14 January 2005

Abstract

We have carried out an investigation of the properties of low redshift EIS clusters using both spectroscopy and imaging data. We present new redshifts for 738 galaxies in 21 ESO Imaging Survey (EIS) Cluster fields. We use the “gap”-technique to search for significant overdensities in redshift space and to identify groups/clusters of galaxies corresponding to the original EIS matched filter cluster candidates. In this way we spectroscopically confirm 20 of the 21 cluster candidates with a matched-filter estimated redshift . We have now obtained spectroscopic redshifts for 34 EIS cluster candidates with (see also Hansen et al. 2002, A&A, 388, 1; Olsen et al. 2003, A&A, 409, 439). Of those we spectroscopically confirm 32 with redshifts ranging from to 0.283. We find that: 1) the velocity dispersions of the systems range from  km s^-1 to  km s^-1, typical of galaxy groups to rich clusters; 2) richnesses corresponding to Abell classes ; and 3) concentration indices ranging from to . From the analysis of the colours of the galaxy populations we find that 60% of the spectroscopically confirmed systems have a “significant” red sequence. These systems are on average richer and have higher velocity dispersions. We find that the colour of the red sequence galaxies matches passive stellar evolution predictions.