The core structure of galaxy clusters from gravitational lensing

We examine gravitational lensing constraints on the structure of galaxy clusters and compare them with the results of cosmological N-body simulations of cluster formation in cold dark matter -dominated universes. We find that cluster core masses, as measured by the observed location of giant tangential arcs, generally exceed those of dark matter halos of similar velocity dispersion. The magnitude of the discrepancy is a strong function of cluster mass. Arc properties in the most massive clusters in the sample (i.e., those with velocity dispersion σ ∼ 1500-2000 km s^-1) are essentially consistent with the N-body predictions. On the other hand, giant arcs in σ ∼ 1000 km s^-1 clusters can be reconciled with cold dark matter cluster halos only if their lensing power, i.e., central surface mass density, has been increased substantially by the presence of a massive (∼ 3 × 10¹² h^-1 M⊙) central galaxy and of significant substructure. Best agreement is found if the mass of the central galaxy and the effects of substructure are approximately independent of cluster mass. Massive central galaxies with steep inner density profiles are also needed to explain 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 of these results, but at present the data set available is too limited to have a significant impact on these conclusions. Our results depend only weakly on the cosmological model adopted and suggest that structural parameters of clusters derived from strong lensing studies cannot usefully constrain the values of cosmological parameters.