Origami: Delineating Halos Using Phase-Space Folds

We present the origami method of identifying structures, particularly halos, in cosmological N -body simulations. Structure formation can be thought of as the folding of an initially flat three-dimensional manifold in six-dimensional phase space. origami finds the outer folds that delineate these structures. Halo particles are identified as those that have undergone shell-crossing along three orthogonal axes, providing a dynamical definition of halo regions that is independent of density. origami also identifies other morphological structures: particles that have undergone shell-crossing along 2, 1, or 0 orthogonal axes correspond to filaments, walls, and voids, respectively. We compare this method to a standard friends-of-friends halo-finding algorithm and find that origami halos are somewhat larger, more diffuse, and less spherical, though the global properties of origami halos are in good agreement with other modern halo-finding algorithms.