A dynamical instability of bars in disk galaxies

Abstract

STRONG, rapidly rotating, persistent bars readily form in numerical simulations of initially axisymmetric disk galaxies^1–4. The global dynamical instability responsible for this behaviour is a great embarrassment to the subject of galactic dynamics, as most galaxies in the sky do not possess a strong bar⁵; the suggestion³ that the dark matter content of galaxies might resolve this discrepancy is unattractive⁶. Here we present the results of three-dimensional simulations which show that the bar is dynamically unstable to buckling out of the galactic plane (the majority of previous disk galaxy simulations have been strictly two-dimensional). Stars acquire large motions normal to the plane, giving the bar a peanut shape through a mechanism that we suggest to be the fire-hose instability^7–9. Bars may be weakened or even destroyed by this instability; thus the fraction of disk galaxies containing strong bars today could be lower than the fraction in which they have formed in the past. The instability may also account for the peanut morphology of many galaxies, and because it leads to a less flattened stellar system with increased central density, it may play a part in the formation of galactic bulges.