EDP Sciences
Free access
Volume 380, Number 2, December III 2001
Page(s) 460 - 470
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20011457

A&A 380, 460-470 (2001)
DOI: 10.1051/0004-6361:20011457

On the nature of the EIS candidate clusters: Confirmation of $z\lesssim$ 0.6 candidates

L. F. Olsen1, C. Benoist2, L. da Costa3, M. Scodeggio4, H. E. Jørgensen1, S. Arnouts3, S. Bardelli5, A. Biviano6, M. Ramella6 and E. Zucca5

1  Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
2  Observatoire de la Côte d'Azur, BP 229, 06304 Nice Cedex 4, France
3  European Southern Observatory, Karl-Schwartzschild-Strasse 2, 85748 Garching bei München, Germany
4  Istituto di Fisica Cosmica, via Bassini 15, 20133 Milano, Italy
5  Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
6  INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo, 11, 34131 Trieste, Italy

(Received 3 September 2001 / Accepted 15 October 2001)

We use publicly available V-band imaging data from the wide-angle surveys conducted by the ESO Imaging Survey project (EIS) to further investigate the nature of the EIS galaxy cluster candidates. These were originally identified by applying a matched-filter algorithm which used positional and photometric data of the galaxy sample extracted from the I-band survey images. In this paper, we apply the same technique to the galaxy sample extracted from V-band data and compare the new cluster detections with the original ones. We find that ~75% of the low-redshift cluster candidates ( $z\lesssim0.6$) are detected in both passbands and their estimated redshifts show good agreement with the scatter in the redshift differences being consistent with the estimated errors of the method. For the "robust" I-band detections the matching frequency approaches ~85% . We also use the available (V-I) color to search for the red sequence of early-type galaxies observed in rich clusters over a broad range of redshifts. This is done by searching for a simultaneous overdensity in the three-dimensional color-projected distance space. We find significant overdensities for ~75% of the "robust" candidates with $z_I\lesssim0.6$. We find good agreement between the characteristic color associated to the detected "red sequence" and that predicted by passive evolution galaxy models for ellipticals at the redshift estimated by the matched-filter. The results presented in this paper show the usefulness of color data, even of two-band data, to both tentatively confirm cluster candidates and to select possible cluster members for spectroscopic observations. Based on the present results, we estimate that ~150 EIS clusters with $z_I\lesssim0.6$ are real, making it one of the largest samples of galaxy clusters in this redshift range currently available in the southern hemisphere.

Key words: galaxies: clusters: general -- large-scale structure of Universe -- cosmology: observations

Offprint request: L. F. Olsen, lisbeth@astro.ku.dk

© ESO 2001