Efficient N-body realisations of axisymmetric galaxies and halos

We present an efficient method for building equilibrium multi-component galaxies with non-spherical haloes and bulges. The gist of our approach is to tailor the velocity ellipsoid directly to the geometry of the mass distribution. Thus we avoid computing the anisotropic velocity dispersions which leads to large savings in the cpu budget. The computational time of the algorithm for triaxial equilibria scales linearly with the number of particles, N. The approximate solution to the velocity field causes structural relaxation: tests with N = 50,000 show that fluctuations of the inertia tensor (not exceeding the 10% level) disappear within one half of a revolution at the half-mass radius. At later times equilibrium properties settle to values close to those sought from the initial conditions. A disc component is then added as described by Hernquist [ApJS, 86 (1993) 389]. Incorporating the above algorithm to his code BUILDGAL, test runs show that the stability of the disc against vertical heating is not substantially modified using our method. The code, MAGALIE, is made generally available (contact the corresponding author). (C) 2001 Elsevier Science B.V. All rights reserved.