Adding Long‐Wavelength Modes to anN‐Body Simulation

We present a new method to add long wavelength power to an evolved $N$-bodysimulation, making use of the Zel'dovich (1970) approximation to changepositions and velocities of particles. We describe the theoretical framework ofour technique and apply it to a P$^3$M cosmological simulation performed on acube of $100$ Mpc on a side, obtaining a new ``simulation'' of $800$ Mpc on aside. We study the effect of the power added by long waves by mean of severalstatistics of the density and velocity field, and suggest possible applicationsof our method to the study of the large-scale structure of the universe.Comment: Revised version, shortened. 15 pages without figures. Accepted for publication in the Astrophysical Journal. Paper and 11 Figures available as .ps.gz files by anonymous ftp at ftp://ftp.mpa-garching.mpg.de/pub/bepi/MA