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 $\Lambda$CDM model and have masses of $M_{200} \sim 10^{14}
h^{-1} M_{\odot}$ and $M_{200} \sim 10^{15} h^{-1} M_{\odot}$ in the two
samples. We limit our analysis to substructures identified in the simulation
with masses larger than $10^{13} h^{-1} M_{\odot}$. With mock redshift surveys
with 200 galaxies within $3R_{200}$, (1) the caustic technique recovers $\sim
30-50$\% of the real substructures, and (2) $\sim 15-20$\% 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.
