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Astron. Astrophys. 343, L29-L34 (1999)

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1. Introduction

Growing evidence for the existence of clusters at [FORMULA] and beyond makes the identification and study of these systems of great interest for probing galaxy evolution and cosmological models. However, the number of confirmed systems at these high redshifts is currently very small, precluding any robust statistical analysis. The largest sample of spectroscopically confirmed clusters has been selected from ROSAT deep X-ray observations (Rosati et al. 1998, Rosati 1998), while a few other [FORMULA] clusters have been discovered in the surroundings of strong radio sources (e.g., Dickinson 1996; Deltorn et al. 1997), or using infrared observations (e.g., Stanford et al. 1997). Although X-ray and infrared searches are very effective in identifying real clusters, their ability to cover large areas of the sky is presently limited, and these methods are not likely to produce large samples of very distant clusters in the short-term. On the other hand, with the advent of panoramic CCD imagers, optical wide-angle surveys have become competitive in identifying cluster candidates up to [FORMULA]. Examples of such surveys, suitable for cluster searches, include those of Postman et al. (1996), Postman et al. (1998) and the ESO Imaging Survey (EIS, Renzini & da Costa 1997), which cover 5, 16 and 17 square degrees, respectively, reaching [FORMULA]. These surveys are currently being used for systematic searches of galaxy cluster candidates employing objective matched-filter algorithms (e.g., Postman et al. 1996). In the case of the EIS project, about 300 candidates have been identified, over the redshift interval [FORMULA], out of which 79 are estimated to have [FORMULA] (Olsen et al. 1998a, b; Scodeggio et al. 1998). However, only with additional observations can these optically-selected high-redshift candidates be confirmed. Establishing the global success rate of this technique (and its possible redshift dependence) is extremely important for the design of future wide-field optical imaging surveys. Indeed, these surveys may play a major role in significantly increasing the number of known distant clusters, thus making them useful tools for probing the high-z universe.

As a test case, two EIS cluster candidates identified in EIS patch B (EIS 0046-2930 and EIS 0046-2951; Olsen et al. 1998b), were observed with the VLT Test Camera (VLT-TC) as part of the ESO VLT-UT1 Science Verification (SV; see Leibundgut, De Marchi & Renzini 1998). After the public release of these Science Verification data, fields including the two candidate clusters have been observed at the ESO 3.5m New Technology Telescope (NTT), as part of an ongoing infrared (IR) survey of EIS patch B (Jorgensen et al. 1999). Therefore, we had the opportunity to combine the VLT optical data with the NTT IR data, and to use both optical and IR color-magnitude (CM) diagrams to search for evidence of a "red sequence" of luminous early-type galaxies, typical of populous clusters at low as well as at high redshift (e.g., Bower, Lucey & Ellis 1992; Stanford, Eisenhardt & Dickinson 1998; Kodama et al. 1998). A clear identification of this sequence would provide strong support to the reality of the clusters, while allowing an independent estimate of their redshift to be obtained.

In this Letter we briefly describe the various observations and the data reduction in Sect. 2; in Sect. 3 we present our results; and in Sect. 4 we summarize our conclusions.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999