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

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4. Summary

We have used deep V- and I-band images of two EIS cluster candidates taken during the ESO VLT-UT1 Science Verification observations to investigate the reality of these clusters. The VLT data were complemented by infrared data taken with SOFI at the NTT. Optical, IR, and optical-IR CM-diagrams have been constructed to search for the presence of the red-sequence typical of bright early-type galaxies in clusters.

In the case of EIS 0046-2930 we find a well-defined sequence at [FORMULA] and [FORMULA]. These galaxies are also concentrated relative to the background suggesting the existence of a cluster at [FORMULA]. The evidence for the other candidate, EIS 0046-2951, is less compelling even though we find a sequence at [FORMULA] and [FORMULA], leading to an estimated redshift of [FORMULA], consistent with its original estimate. However, the scatter in the CM-diagrams is large and the density contrast of the "cluster" relative to the background smaller. In any case, a final conclusion on whether these systems are real physical associations at high-redshift must await spectroscopic observations.

These results demonstrate once again the importance of infrared data in locating high-redshift clusters. However, the small field of view of present IR detectors makes large solid angle IR surveys very expensive in terms of telescope time. On the other hand, wide angle optical surveys can efficiently produce a great number of high redshift candidates, but with a major fraction of them which may turn to be spurious after time-consuming spectroscopic follow up. The present experiment is an attempt at exploring a hybrid approach, in which the optically selected candidates are first imaged in the IR before being considered for spectroscopic follow up at a large telescope such as the VLT. Besides providing a first verification of the candidate clusters, the IR images can then be used to search for clusters at higher redshift. The overall efficiency of this strategy remains to be empirically determined, e.g. for the actual complement of ESO telescopes and instruments, and the present paper represents a first step in this direction.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999