1 Introduction

 

The discovery of clusters of galaxies at high redshift has motivated efforts of compiling lists of candidates for follow-up observations with 8 m-class telescopes. The interest in studying these systems spans a broad range of topics and searching for them was identified as one of the primary goals of the ESO Imaging Survey (EIS, Renzini & da Costa 1997), a moderately deep wide-field imaging survey conducted at the 3.5 m New Technology Telescope (NTT) at La Silla. The main requirements for the cluster search were: 1) to produce a list of candidates large enough to meet the needs of potential VLT programs; 2) to span a broad range of redshifts; 3) to cover a wide range of right ascension thereby allowing the selection of targets year round; 4) to minimize as much as possible spurious detections. These requirements dictated to a large extend the observing strategy adopted by EIS, including the partition of the survey area into four fields, and the preference given to I-band observations in the second-half of the program. While searches at other wavelengths may provide less contaminated and better defined samples (e.g.,and X-ray searches), optical searches have the advantage of producing large samples at a faster rate than any other search method, especially with the advent of CCD wide-field imagers.

As stated in Olsen et al. (1999a; Paper II) the main goal of the EIS cluster search program is to timely provide the astronomical community with a list of cluster candidates that can be used as individual targets for follow-up observations in the Southern Hemisphere, especially with the VLT. It must be emphasized that it is not the intention of this search program to provide a complete and well-defined sample for statistical studies, since such analysis is beyond the scope of the present effort.

The original aim of EIS was to observe in V- and I-band about 20 square degrees in four different patches of the sky (see Renzini & da Costa 1997, and also Nonino et al. 1999; Paper I). However, as described in earlier papers (Paper I, Prandoni et al. 1999; Paper III) the first-half of the program was severely compromised by bad weather, and therefore in the second-half I-band observations covering EIS patches C and D were given priority. The data for these patches are far superior to those obtained earlier in patches A and B and the full coverage of the pre-selected areas was possible, yielding a total area of about 12 square degrees (Benoist et al. 1999; Paper VI). In this paper the list of cluster candidates found in these regions by using the cluster finding pipeline described in Paper II is presented. These results extend the candidate cluster sample presented in Paper II and by Olsen et al. (1999b; Paper V), providing targets nearly year round.

In Sect. 2 some aspects of the data relevant to the application of the cluster detection algorithm are discussed. In Sect. 3 a list of 257 candidate clusters is presented and their properties are compared with those of other candidates detected in ESI patches A and B and in the Palomar Distant Cluster Survey (PDCS, Postman et al. 1996), currently the only comparable survey. A brief summary of the results is presented in Sect. 4.