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Astron. Astrophys. 345, 363-368 (1999)

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1. Introduction

The ESO Imaging Survey (EIS; Renzini & da Costa 1997) is the outgrowth of a concerted effort between ESO and its community to carry out an imaging survey and provide candidate targets for the rapidly approaching first year of regular operation of the VLT. The main science goals of the survey have been described earlier (Nonino et al. 1998, Paper I, Prandoni et al. 1998, Paper III). These papers also include a detailed account of the survey observing strategy, and of the data obtained for two of the four areas covered by the survey (EIS patches A and B, at [FORMULA], [FORMULA] and [FORMULA], [FORMULA], respectively). One of the main goals of EIS is the compilation of a list of candidate clusters of galaxies, spanning a large redshift range. Following the timetable recommended by the EIS Working Group, lists of cluster candidates in the various areas covered by the survey are being prepared and made public as soon as they become available. In addition, an interface with the ESO Science Archive has been created, allowing image postage stamps of the candidates to be extracted for visual inspection and preparation of finding charts (see "http://www.eso.org/eis").

A preliminary catalog containing 35 cluster candidates identified in EIS patch A, was presented in a previous paper (Olsen et al. 1998, hereafter Paper II). As emphasized in that paper, the goal of the EIS cluster search has been to prepare a list of candidates for follow-up observations and not to produce a well-defined sample for statistical analysis. For the cluster search the matched filter algorithm discussed by Postman et al. (1996) was chosen, since it has already been tested and used to analyze similar data. Moreover, by using the same algorithm the results obtained here can be easily compared to those obtained by those authors. Since the data are public, other groups may produce their own catalogs using different methods. A comparison of various catalogs will be instructive in evaluating the strengths and weaknesses of different algorithms.

While completeness is an important issue for statistical studies, the main concern of the EIS cluster search program is the reliability of the candidates, and therefore the minimization of the number of false detections, to optimize any future follow-up work. To minimize contamination by spurious detections, the analysis has been restricted to the most uniform surveyed areas. Moreover, the parameters adopted in searching for candidates have been conservatively chosen, based on the results of an extensive set of simulations, to minimize the contamination by noise peaks (Paper II).

Another way of further testing the reality of the detections is to use data in different passbands. In Paper II candidate clusters were detected using only I-band data. Currently, however, V-band galaxy catalogs are available for [FORMULA] 2.9 square degrees. The availability of these data allows one to detect clusters in the V-band and cross-identify them with those detected in the I-band. The color information can be used: 1) to confirm the reality of detections directly from the cross-identification; 2) to confirm detections by identifying the expected sequence of cluster early-type galaxies in the color-magnitude (CM) diagram; 3) to use the CM relation to independently check the estimated cluster redshift and select targets for follow-up spectroscopy.

The goals of the present paper are to add to the list of EIS I-band cluster candidates those detected in patch B and to investigate what additional information the V-band data provide over the region where V and I images overlap. The data are described in Sect. 2; Sect. 3 describes the cluster candidate catalog and the investigation of the color information; the conclusions are summarized in Sect. 4.

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© European Southern Observatory (ESO) 1999

Online publication: April 19, 1999