1. Spontaneous Spiral Structure

Swing amplification of particle noise can bring forth trailing multi-armed spiral patterns. Shown here is an N-body experiment with a central bulge (yellow), an exponential disk (blue), and a dark halo (red). Apart from Poissonian fluctuations due to particle noise, this disk is initially featureless. Later frames, separated by about half a rotation period at three exponential scale lengths, show the development of transient spiral patterns.

2. Tidal Spiral Structure

Tides between galaxies provoke a two-sided response. Such perturbations, if further swing-amplified in differentially-rotating disks, may produce striking `grand-design' spiral patterns. In the experiment shown here, an artificial tide was applied by taking the unperturbed disk above and instantaneously replacing each x velocity with

        v_x <- v_x + k x ,
where k is a constant used to adjust the strength of the perturbation. No perturbation was applied to the y and z velocities. Frames made after 1.5 rotation periods show the development of an open, two-armed spiral pattern which becomes more tightly wound with time.

Joshua E. Barnes (barnes@zeno.ifa.hawaii.edu)

Last modified: September 3, 1996