Fast identification of bound structures in large N-body simulations

We present an algorithm that is designed to allow the efficient identification and preliminary dynamical analysis of thousands of structures and substructures in large N-body simulations. First, we utilize 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 catalogue of separate energetically bound substructures, we check to see which of these are energetically bound to adjacent substructures. For such bound complex subhaloes, 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 subhaloes can contain more than one density maximum, but the scheme is stable: starting with a small smoothing length initially produces small structures that 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 that are using the TPM algorithm by Bode, Ostriker & Xu and one on a simulated halo by Diemand, Moore & Stadel. For all these haloes, we find about 5-8 per cent of the mass in substructures.