Quantifying substructure in galaxy clusters

Substructure in galaxy clusters can be quantified with the robust a statist ics (Dressler and Shectman 1988) which uses velocity kinematics and sky pro jected positions. We test its sensitivity using dissipationless numerical s imulations of cluster formation. As in recent observations, about 30% of th e simulated clusters show substructure, but the exact percentage depends on the chosen limit for defining substructure, and a better discriminator is the distribution function of the Delta statistics. The Dressler-Shectman st atistics correlate well with other subcluster indicators, but with large sc atter due to its sensitivity to small infalling groups and projection effec ts.