On the evolution of shape in N-body simulations

A database on shape evolution of direct N-body models formed out of cold, d issipationless collapse is generated using GRAPE and HARP special purpose c omputers. Such models are important to understand the formation of elliptic al galaxies. Three dynamically distinct phases of shape evolution were foun d, first a fast dynamical collapse which gives rise to the radial orbit ins tability (ROI) and generates at its end the maximal triaxiality of the syst em. Subsequently, two phases of violent and two-body shape relaxation occur , which drive the system first towards axisymmetry, finally to spherical sy mmetry (the final state, however, is still much more concentrated than the initial model). In a sequence of models the influence of numerical and phys ical parameters, like particle number, softening, initial virial ratio, tim estep choice, different N-body codes, are examined. We find that an imprope r combination of softening and particle number can produce erroneous result s. Selected models were evolved on the secular timescale until they became spherically symmetric again. The secular shape relaxation time scale is sho wn to agree very well with the two-body relaxation time, if softening is pr operly taken into account for the latter. Finally we argue, that the interm ediate phase of violent shape relaxation after collapse is induced by stron g core oscillations in the centre, which cause potential fluctuations, damp ening out the triaxiality.