THE DISTRIBUTION OF DARK-MATTER IN DISTANT CLUSTER-LENSES - MODELING A-370

We investigate the distribution of the dark matter in the distant clus ter of galaxies A 370 (z=0.375) on the basis of our new, ultra-deep, s ubarcsecond seeing CCD images showing evidence for three merging parts of the giant arc and of some new multiple image candidates. The numbe r of constraints is thus considerably increased, allowing a reliable d etermination of the parameters of the assumed double elliptical potent ial. The net result is a remarkable quality of fit of the complex stru cture revealed by our new data along the giant arc. The dark matter li es in two clumps centered on the two brightest cluster galaxies, with similar orientation and ellipticity as their two envelopes. The indivi dual line-of-sight velocity dispersion of each clump is around 850 km s-1, the global velocity dispersion being consistent with that measure d for the cluster. The two dark clumps are characterized by small core radii: r(c) congruent-to 100 h-1 kpc. The redshift of the giant arc b eing available, the model is so constrained that it enables accurate d etermination of the redshift of three faint pairs of images, simply by assuming that they are double images of a distant source across the c ritical line at this redshift. In this model of the cluster A 370, as in the one recently obtained for MS2137-23, the distribution of the da rk matter appears to follow the distribution of light and to be highly concentrated on the brightest galaxies. If confirmed, these results s hould give interesting constraints on the formation and evolution of c D clusters of galaxies.