Efficient Methods for Structure Formation

Bridget Falck (JHU)

First, we investigate the use of a logarithmic density variable in estimating the Lagrangian displacement field. The linear and logarithmic density-displacement relations are compared by measuring both of these fields directly in a cosmological /N/-body simulation. We find that the relation between the divergence of the displacement field and the density is significantly tighter with a logarithmic density variable, especially at low redshifts and for very small (~2Mpc/h) smoothing scales, and we find that grid-based methods are more reliable than the tessellation-based method of calculating both the density and the divergence fields. We then present the ORIGAMI method of identifying structures, particularly halos, in cosmological /N/-body simulations. Halo particles are identified as those that have undergone shell-crossing along 3 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 and more diffuse, though the global properties of ORIGAMI halos are in good agreement with other modern halo-finding algorithms. Finally, we briefly describe the currently-running Indra suite of cosmological simulations and the possibilities enabled by large-scale simulations in a database.