Fast Identification of Bound Structures in Large N-body Simulations
  We present an algorithm which is designed to allow the efficient identification and preliminary dynamical analysis of thousands of structures and substructures in large N-body simulations. First we utilise a refined density gradient system (based on DENMAX) to identify the structures, and then apply an iterative approximate method to identify unbound particles, allowing fast calculation of bound substructures. After producing a catalog of separate energetically bound substructures we check to see which of these are energetically bound to adjacent substructures. For such bound complex subhalos, we combine components and check if additional free particles are also bound to the union, repeating the process iteratively until no further changes are found. Thus our subhalos can contain more than one density maximum, but the scheme is stable: starting with a small smoothing length initially produces small structures which must be combined later, and starting with a large smoothing length produces large structures within which sub-substructure is found. We apply this algorithm to three simulations. Two which are using the TPM algorithm by Bode et al. (2000) and one on a simulated halo by Diemand (2004). For all these halos we find about 5-8% of the mass in substructures. 
Jochen Weller, Jeremiah P Ostriker, Paul Bode and Laurie Shaw