Spiral Instabilities in N-body Simulations I: Emergence from Noise

The origin of spiral patterns in galaxies is still not fully understood. Similar features also develop readily in N-body simulations of isolated cool, collisionless disks, yet even here the mechanism has yet to be explained. In this series of papers, I present a detailed study of the origin of spiral activity in simulations in the hope that the mechanism that causes the patterns is also responsible for some of these features galaxies. In this first paper, I use a suite of highly idealized simulations of a linearly stable disk that employ increasing numbers of particles. While the amplitudes of initial non-axisymmetric features scale as the inverse square-root of the number of particles employed, the final amplitude of the patterns is independent of the particle number. I find that the amplitudes of non-axisymmetric disturbances grow in two distinct phases: slow growth occurs when the relative overdensity is below ~2%, but above this level the amplitude rises more rapidly. I show that all features, even of very low amplitude, scatter particles at the inner Lindblad resonance, changing the distribution of particles in the disk in such a way as to foster continued growth. Stronger scattering by larger amplitude waves provokes a vigorous instability that is a true linear mode of the modified disk.