A computer-generated galaxy model with long-lived two-armed spiral structure

A long-lived two-armed spiral has been generated in an N-body computer simulation of a galaxy with a static bulge and halo and an active disk composed of 60,000 particles. The spiral lasts for about three pattern revolutions without severe distortion and persists for at least two more revolutions with distortions and bifurcations resulting from an increasingly clumpy ISM. This suggests that two-armed grand design spirals in nonbarred noninteracting galaxies can be long-lived if star formation and other heat sources not present in the simulation maintain a steady interstellar medium. 40 refs