Tools and Techniques for N-body Simulations

Computational Astrophysics research often requires very powerful machines, and very sophisticated codes. In stellar dynamics simulations, the development of the treecode, and the realisation of the GRAPE special purpose device, have contributed dramatically to the progress of scientific research. The pseudoparticle approach, where a multipole expansion is expressed in terms of a particle distribution, allows to improve the tree-code accuracy at a limited computational cost. Moreover, it is suitable for making full use of the GRAPE. We study the error behaviour of this approach with respect to changing physical distributions. Thus we introduce improvements that reduce the errors. Furthermore we present an extension of the pseudo-particle scheme, where pseudo-particles are not fixed in space, but move following the physical particle distribution. This extension decreases the computational overhead due to pseudo-particle recomputation, and optimises the scheme for the use on GRAPE and on parallel systems. We also study the efficiency at which a black hole spirals-in to the Galactic centre, using the direct N -body method running on the GRAPE, a tree algorithm and a particle-mesh technique. The three different techniques are in excellent agreement. The black hole infall, within the set of parameters that we consider, is not influenced by the granularity of the N -body system.