The core structure of galaxy clusters from gravitational lensing

We examine gravitational lensing constraints on the structure of galaxy clu sters and compare them with the results of cosmological N-body simulations of duster formation in cold dark matter -dominated universes. We find that cluster core masses, as measured by the observed location of giant tangenti al arcs, generally exceed those of dark matter halos of similar velocity di spersion. The magnitude of the discrepancy is a strong function of cluster mass. Are properties in the most massive clusters in the sample (i.e., thos e with velocity dispersion sigma similar to 1500-2000 km s(-1)) are essenti ally consistent with the N-body predictions. On the other hand, giant arcs in sigma similar to 1000 km s(-1) clusters can be reconciled with cold dark matter cluster halos only if their lensing power, i.e., central surface ma ss density, has been increased substantially by the presence of a massive ( similar to 3 x 10(12) h(-1) M.) central galaxy and of significant substruct ure. Best agreement is found if the mass of the central galaxy and the effe cts of substructure are approximately independent of cluster mass. Massive central galaxies with steep inner density profiles are also needed to expla in a clear trend, observed in our data set, between the radial thickness of giant tangential arcs and the velocity dispersion of the lensing cluster. The position and redshift of radial arcs may be used as independent tests o f these results, but at present the data set available is too limited to ha ve a significant impact on these conclusions. Our results depend only weakl y on the cosmological model adopted and suggest that structural parameters of clusters derived from strong lensing studies cannot usefully constrain t he values of cosmological parameters.