Spiral instabilities in N-body simulations – II. Emergence from noise

Abstract

An earlier paper presented the potentially significant discovery that disturbances in simplified simulations of a stellar disc model that was predicted to be stable in linear theory grew to large amplitude over a long period of time. The ultimate appearance of true instabilities was attributed to non-linear scattering by a succession of collective waves excited by shot noise from the finite number of particles. The paper concluded that no finite number of particles, however large, could mimic a smooth disc. As this surprising finding has been challenged as an artefact of the numerical scheme employed, we here present a new calculation of the same model using a different grid geometry that confirms the original behaviour.