[BBR2002] 11 , the SIMBAD biblio

We present the results of a new spectroscopic and photometric survey of the hot X-ray cluster 1E 0657-56, at z=0.296. We determine the presence of a low velocity dispersion subcluster, which is offset from the main cluster position by 0.7Mpc and ≃600km/s. We determine the virial masses and total luminosities of the cluster and its subcluster, and solve for the two-body dynamical model. With additional constraints from the results of the analysis of the cluster X-ray emission by Markevitch et al. (2002ApJ...567L..27M), we find that the subcluster passed through the cluster centre ≃0.15Gyr ago. Taken at face value the mass of the subcluster is typical of a loose group. It is however difficult to establish the pre-merger mass of the colliding system. We provide tentative evidence that the subcluster is in fact the remnant core of a moderately massive cluster, stripped by the collision with 1E 0657-56. The main cluster dynamics does not seem to have suffered from this collision. On the contrary, the cluster X-ray properties seem to have been significantly affected. We also discuss the effect of the subcluster collision in relation to starburst events and the cluster radio halo.