We have developed a cosmological N-body code which can simulate unprecedently large number of massive particles. This code is based on the Particle-Mesh scheme, and utilize the recent fast I/O devices to swap all variables. Using the new code we have simulated the formation and evolution of structures at high redshifts in the standard Cold Dark Matter (CDM) cosmogony. A simulation evolving 1024^3 particles on a 2048^3 mesh with the initial standard CDM power spectrum is being made. This is the first billion particle cosmological simulation with initial conditions representing the theoretical model over the widest range of space. A smaller, but still very large CDM simulation with 512^3 particles on a 1024^3 mesh has been completed. We have found that the galaxy-scale CDM halos with diameters of tens of kpcs undergo complete collapse before redshift 10. Our results clearly indicate that galactic and subgalactic structures have formed far before redshift 5 which is the present upper limit to the epoch of observed structures. We emphasize that the non-linear evolution of the galactic and subgalactic-scale structures starts as early as z ~ 50, and that cosmological simulations must start at such high redshifts. A high mass resolution is also indispensable to accurately represent the theoretical model in the initial conditions down to subgalactic scales, and to correctly study the subsequent formation and evolution of structures through hierarchical clustering.

• Park Changbom(Department of Astronomy, Seoul National University)