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Astron. Astrophys. 360, 861-870 (2000)

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5. Discussion and conclusions

We obtain 67 new redshifts for galaxies in six EIS candidate cluster fields. Based on these data, we establish the existence of real systems in redshift space in the direction of four of these candidate clusters. The reality of the systems is established at [FORMULA]% confidence level, and in two cases, at [FORMULA]%. The redshift overdensities, coupled with the 2-d overdensities detected by the use of the matched-filter algorithm, strongly supports the reality of four of the six examined EIS clusters. These 4 clusters add to the other two spectroscopically confirmed EIS clusters (da Costa et al. 1999).

Two of the four z-systems have a median redshift in good agreement with the matched-filter estimate for the EIS cluster redshift ([FORMULA] median[FORMULA]). The other two have significantly lower redshifts (median[FORMULA], 0.236 vs. [FORMULA]).

Taken at face value, these results suggest that, in several cases, the matched-filter algorithm over-estimates the mean cluster redshift by a large amount. However, it is quite possible that in some cases we have not detected the EIS cluster, but a foreground galaxy system projected along the same line-of-sight of the cluster. Similarly, it is difficult to conclude about the reality of the EIS clusters where we do not detect any real redshift system.

In particular, we note that in the field of EIS0540-2418 we have a marginal detection (92% probability) of the galaxy set 2d at median[FORMULA] (see Table 3), in fair agreement with the matched-filter estimate of the cluster redshift, [FORMULA]. We also note that the cluster EIS0533-2353 has [FORMULA], larger than any of our galaxy sets. This suggests that it could have escaped detection because our observations were not deep enough. In fact, da Costa et al. (2000) suggest that all our six EIS cluster candidates could be real, based on the analysis of the colour-magnitude diagrams for galaxies in the cluster fields.

We conclude that our spectroscopic confirmation rate must be considered as a lower limit. If at least one third of the EIS clusters in the redshift range sampled by our observations are real, there are more than 25 EIS clusters with [FORMULA] in the range 0.5-0.7. This sample is large enough for the derivation of the properties of clusters at intermediate to high redshifts.

Optical selection of clusters of galaxies at high redshifts is a necessary complementary approach to X-ray selection. While X-ray selection tends to detect only rich Abell-like clusters, optically selected cluster samples contain a large number of poor clusters. In fact, the space density of PDCS clusters is five times higher than that of rich Abell clusters (Holden et al. 1999a), and very few PDCS clusters are X-ray bright (Holden et al. 1997). Consistently, the velocity dispersions of our two systems with [FORMULA] galaxy redshifts (system 4c, at [FORMULA] and system 6c, at [FORMULA], see Table 3) are [FORMULA] km s-1, typical of low-richness clusters ([FORMULA], see Girardi et al. 1993).

With the current and near-future ground-based facilities for wide-field optical and near-infrared imaging, we can expect a rapid increase in the samples of optically selected clusters. Currently, our spectroscopic sample only comprises [FORMULA]% of all the clusters in the two patches C and D, and in the (estimated) redshift range 0.5-0.7. We plan to extend our sample in forthcoming observing runs. Confirmed EIS clusters at high redshift will be the natural targets of VLT observations aimed at determining their dynamical properties.

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000
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