IDENTIFICATION OF GALAXY CLUSTER SUBSTRUCTURES WITH THE CAUSTIC METHOD

We investigate the power of the caustic technique for identifying substructures of galaxy clusters from optical redshift data alone. The caustic technique is designed to estimate the mass profile of galaxy clusters to radii well beyond the virial radius, where dynamical equilibrium does not hold. Two by-products of this technique are the identification of the cluster members and the identification of the cluster substructures. We test the caustic technique as a substructure detector on two samples of 150 mock redshift surveys of clusters; the clusters are extracted from a large cosmological N-body simulation of a ΛCDM model and have masses of M200 ∼ 1014h-1M⊙ and M200 ∼ 1015h-1M⊙ in the two samples. We limit our analysis to substructures identified in the simulation with masses larger than 1013h-1M⊙. With mock redshift surveys with 200 galaxies within 3R200, (1) the caustic technique recovers ∼30%-50% of the real substructures, and (2) ∼15%-20% of the substructures identified by the caustic technique correspond to real substructures of the central cluster, the remaining fraction being low-mass substructures, groups or substructures of clusters in the surrounding region, or chance alignments of unrelated galaxies. These encouraging results show that the caustic technique is a promising approach for investigating the complex dynamics of galaxy clusters.