4 Summary

We have reported 104 radial velocity measurements performed with the MEFOS multifiber spectrograph at the 3.6-m ESO telescope for 99 galaxies in a region of $30\hbox{$^\prime$}$ around the center of the cluster A3733. To augment this data, we have combined the MEFOS measurements with 39 redshifts measured by Stein (1996) with the OPTOPUS instrument at the same telescope. This has given a final dataset with a total of 112 entries in the field of A3733. Radial velocities have been then supplemented by COSMOS $b_{\rm J}$ magnitudes and accurate sky positions in order to investigate the kinematics and structure of the central regions of the cluster.

From a sample containing 74 strict cluster members, we have derived a heliocentric systemic velocity for A3733 of $11\,653^{+74}_{-76}$ $\mbox{km\,s}^{-1}$,resulting in a $\overline{z}_{\rm CMB}$ of 0.0380, and a velocity dispersion of 614⁺⁴²_-30 $\mbox{km\,s}^{-1}$, in good agreement with the estimates by Stein (1997) from the OPTOPUS data alone. Statistical tests relying exclusively on the distribution of observed velocities have yield suggestive indication of the possible kinematical complexity of A3733, especially when applied to a nearly complete magnitude-limited ($b_{\rm J}$ $\leq 18$) sample of cluster members. Despite this result, two powerful substructure tests that incorporate spatial information have failed to detect in this latter sample any statistically significant evidence of clumpiness in the galaxy component, in agreement with the findings of a previous study based on a spatially less extended and less complete dataset. Given that the sensitivity of the spatial substructure tests we have used is reduced when the subunits are seen with small projected separations, the results of the present study cannot exclude, however, the possibility that the signs of kinematical complexity detected in the velocity histogram of A3733 might be due to the existence of galaxy subcondensations superposed along the line-of-sight.

Acknowledgements

This work has been supported by the Dirección General de Investigación Científica y Técnica, under contract PB96-0173. P.S. acknowledges partial support from a research network grant by the Commission of the European Communities.