EDP Sciences
Free access
Volume 435, Number 3, June I 2005
Page(s) 781 - 798
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20041850

A&A 435, 781-798 (2005)
DOI: 10.1051/0004-6361:20041850

Multi-object spectroscopy of low redshift EIS clusters

III. Properties of optically selected clusters
L. F. Olsen1, C. Benoist2, L. da Costa3, L. Hansen1 and H. E. Jørgensen1

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 )

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 $z_{\rm MF}=0.2$. We have now obtained spectroscopic redshifts for 34 EIS cluster candidates with $z_{\rm MF}=0.2$ (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 z=0.064 to 0.283. We find that: 1) the velocity dispersions of the systems range from $\sigma_{\rm v}\leq130$ km s-1 to $\sigma_{\rm v}=1200$ km s-1, typical of galaxy groups to rich clusters; 2) richnesses corresponding to Abell classes $R \leq1$; and 3) concentration indices ranging from C=0.2 to C=1.2. 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.

Key words: galaxies: clusters: general -- cosmology: observations -- galaxies: distances and redshifts -- galaxies: photometry

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