Multiple mesh techniques for modelling interacting galaxies

In modelling interacting galaxies by particle-mesh techniques one must cope with at least two length scales. The separation of components can exceed 100 kpc and we need to resolve detail on a scale of 1 kpc or less in individual components. The authors developed two techniques to enable them to determine potentials on local fine meshes correctly (within the usual finite difference approximation) without calculating details of the potential in intervening regions. Both techniques depend on the corrective charge procedure of the present Poisson solver. One uses direct FFT based convolution to calculate interaction potentials and is intended specifically for interaction calculations. The other segments a large mesh and uses corrective charges to find potentials on segment boundaries as an intermediate step in finding interior potentials. It may be used for interaction calculations or for solving very large problems on multi-processor configurations