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

Abstract

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 interstellar medium. 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. The key factors contributing to the generation of this pattern are (1) a Q-barrier in the inner region, (2) a mass distribution and rotation curve that allows swing amplification of m = 2 waves with an inner Lindblad resonance inside the Q-barrier, (3) a starcooling routine that keeps the stellar velocity dispersion constant with adiabatic variations allowed, and (4) a substantial amount of cold, dissipating gas in the disk. The stellar pattern extends from the Q-barrier to the outer Lindblad resonance, and the gaseous pattern extends from the region of the inner Lindblad resonance to the outer Lindblad resonance.