Confirming EIS Clusters . Multi - object Spectroscopy

Clusters of galaxies arise from exceptionally high peaks of the primordial fluctuation density field. Their properties as a function of redshift, z, are therefore highly sensitive to the nature of such cosmic fluctuations. It is therefore very important to have a sample of galaxy clusters covering as wide a redshift range as possible. Recently, Olsen et al. (1999) and Scodeggio et al. (1999) have identified clusters in 2D from the I-band images of the ESO Imaging Survey (EIS, see Renzini & da Costa 1997), using the matched filter algorithm of Postman et al. (1996). Very little is known on the performance of this algorithm at z ≥ 0.5, and many of the cluster candidates may not be real. The spectroscopic redshifts and confirmation of cluster candidates in the range 0.5 ≤ z ≤ 0.7 is possible with 4m-class telescopes. Here we report on preliminary results of new spectroscopic observations of six EIS candidate clusters. A complete description of our survey and results will be published in Ramella et al. (in preparation). The selected cluster candidates have estimated redshifts (from the matched filter algorithm) 0.5 ≤ z mf ≤ 0.7. We observed these cluster fields with EFOSC2 at the 3.6 m ESO telescope at La Silla, in Multi-Object Spectroscopy mode, during two nights in February 1999, in average weather conditions and partial moonlight.