5 Conclusions

In this paper we present a catalogue of spectroscopic data for 687 objects in a field of $21\times25\,{\rm arcmin}^2$ around the centre of the cluster of galaxies Cl0024+1654 at $z\sim0.395$, 295 of which lie in the vicinity of the cluster itself, in the range 0.37<z<0.41. The completeness of the sample exceeds 80% around the cluster centre for V<22, dropping to $\sim$70% for $V<\,23$. The mean completeness over the whole field is about 45% down to V=22. This is therefore one of the largest spectroscopic surveys available for a cluster at and the largest at $z\sim0.4$. Apart from redshifts the catalogue lists photometric data and equivalent widths for five lines as well as the strength of the 4000 Å break, important for determining present and recent star forming activity. The catalogue represents a unique database for investigations into the structure of a medium-redshift cluster of galaxies and its environment.

We report the discovery of a binary group of galaxies at $z\simeq0.495$, which includes a gravitational arc candidate. A further overdensity in the redshift distribution at $z\sim0.18$can at present not be attributed to a collapsed structure, although we cannot exclude the possibility that a centre for such a structure exists outside the field covered by our survey. None of the structures seen in the redshift distribution seems to be able to explain the coherent shear signal at $\sim6\hbox{$^\prime$ }$ to the north-east of the projected centre of Cl0024+1654, detected by Bonnet et al. (1994).

The main result of this project is that Cl0024+1654 is not a simple isolated cluster, as has hitherto been assumed in interpreting kinematical, lensing and X-ray data. Instead there is a second, less massive cluster projected onto the centre of the main cluster. The separation of Cl0024+1654 into two components has strong consequences for the interpretation of observational data and should in particular help to resolve the well-known discrepancy between masses determined using different methods for this cluster (Soucail et al. 2000). The structure of Cl0024+1654, a scenario for possible interaction between the main and foreground clusters and the effects of this interaction on the properties of the cluster galaxy populations will be discussed in more detail in the second paper of this series (Czoske et al. 2001). The photometric data obtained in conjunction with this project will be used by Mayen et al. (2001) to investigate the depletion of background objects due to the gravitational lensing effect of Cl0024+1654.

Acknowledgements

OC thanks the European Commission for generous support under grant number ER-BFM-BI-CT97-2471. JPK thanks CNRS for support and acknowledges a European Large Scale Facility grant for the WHT observation. This work was supported by the TMR Network "Gravitational Lensing: New Constraints on Cosmology and the Distribution of Dark Matter'' of the European Commission under contract No. ER-BFM-RX-CT97-0172. TJB wishes to thank Observatoire Midi-Pyrénées for hospitality.