HOP - A NEW GROUP-FINDING ALGORITHM FOR N-BODY SIMULATIONS

We describe a new method (HOP) for identifying groups of particles in N-body simulations. Having assigned to every particle an estimate of i ts local density, we associate each particle with the densest of the N -hop particles nearest to it. Repeating this process allows us to trac e a path, within the particle set itself, from each particle in the di rection of increasing density. The path ends when it reaches a particl e that is its own densest neighbor; all particles reaching the same su ch particle are identified as a group. Combined with an adaptive smoot hing kernel for finding the densities, this method is spatially adapti ve, coordinate-free, and numerically straightforward. One can proceed to process the output by truncating groups at a particular density con tour and combining groups that share a (possibly different) density co ntour. While the resulting algorithm has several user-chosen parameter s, we show that the results are insensitive to most of these, the exce ption being the outer density cutoff of the groups.