Quantifying Substructure Using Galaxy-Galaxy Lensing in Distant Clusters

We present high-resolution mass reconstructions for five massive cluster-lenses spanning a redshift range from z = 0.18 - 0.57 utilizing archival Hubble Space Telescope data and applying galaxy-galaxy lensing techniques. These detailed mass models were obtained by combining constraints from the observed strong and weak lensing regimes. Quantifying the local weak distortions in the shear maps in terms of perturbations induced by the presence of galaxy halos around individual bright early-type cluster member galaxies, we estimate the fraction of mass in the central regions of these clusters that can be associated with small scale mass clumps. This technique enables us to directly map the substructure in the mass range 10^{11} - 10^{12.5} solar masses which we associate with galaxy-scale sub-halos. The determination of the mass spectrum of substructure in the inner regions of these clusters is presented. Constraints are thereby obtained on the masses, mass-to-light ratios and truncation radii for these sub-halos. We find that the fraction of total cluster mass associated with individual sub-halos within the inner 0.5h^{-1} - 0.8h^{-1} Mpc of these clusters ranges from 10 - 20%. Our results have important implications for the survival and evolution of substructure in high density cluster cores and are consistent with the theoretical picture of tidal stripping of galaxy-scale halos in high-density cluster environments as expected in hierarchical Cold Dark Matter dominated structure formation scenarios.