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Astron. Astrophys. 360, 861-870 (2000)


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Spectroscopic confirmation of clusters from the ESO imaging survey *

M. Ramella 1, A. Biviano 1, W. Boschin 1, S. Bardelli 2, M. Scodeggio 3,4, S. Borgani 5,6, C. Benoist 4,7, L. da Costa 4, M. Girardi 8, M. Nonino 1,4 and L.F. Olsen 9

1 Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
2 Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
3 Istituto di Fisica Cosmica "G. Occhialini", via Bassini 15, 20133 Milano, Italy
4 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
5 INFN, Sezione di Trieste, c/o Dipartimento di Astronomia, Università degli Studi di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
6 INFN, Sezione di Perugia, c/o Dipartimento di Fisica dell'Università, via A. Pascoli, 06123 Perugia, Italy
7 DAEC, Observatoire de Paris-Meudon, 5 Pl. J. Janssen, 92195 Meudon Cedex, France
8 Dipartimento di Astronomia, Università degli Studi di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
9 Copenhagen University Observatory, Juliane Maries Vej 30, 2100 Copenhagen, Denmark

Received 23 March 2000 / Accepted 9 May 2000

Abstract

We measure redshifts for 67 galaxies in the field of six cluster candidates from the ESO Imaging Survey (EIS). The cluster candidates are selected in the EIS patches C and D among those with estimated mean redshifts [FORMULA]. The observations were made with EFOSC2 at the 3.6m ESO telescope.

In the six candidate cluster fields, we identify 19 possible sets of 2 to 7 galaxies in redshift space. In order to establish which of the 19 sets are likely to correspond to real dense systems we compare our counts with those expected from a uniform distribution of galaxies with given luminosity function. In order to take into account the effect of the Large Scale Structure, we modulate the probability computed from the luminosity function with random samplings of the Canada-France Redshift Survey.

We find that four out of six candidate EIS clusters are likely to correspond to real systems in redshift space ([FORMULA]% confidence level). Two of these systems have mean redshift in agreement with the redshift estimate given by the matched filter algorithm ([FORMULA]). The other two systems have significantly lower redshifts.

We discuss the implications of our results in the context of our ongoing research projects aimed at defining high-redshift optically-selected cluster samples.

Key words: cosmology: observations – cosmology: large-scale structure of Universe – galaxies: clusters: general – galaxies: distances and redshifts

* Based on observations collected at the European Southern Observatory (La Silla, Chile), Proposal ID: 62.O-0601

Send offprint requests to: A. Biviano (biviano@oat.ts.astro.it)

© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000

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