After 54 years of bar instability studies: a fresh surprise

TL;DR

This study revisits the long-standing problem of bar instability in disk galaxies, revealing that certain stabilizing effects are due to higher-order instabilities, but stability is lost with live halos, highlighting complex dynamics.

Contribution

It uncovers how higher-order instabilities can suppress bar formation in rigid halo models and demonstrates that this suppression does not occur with live halos, challenging previous stability assumptions.

Findings

Higher-order instabilities can suppress bar modes in rigid halo models.

Replacing rigid halos with live halos restores bar instability.

Stability conditions depend on halo dynamics and model specifics.

Abstract

The well-known bar instability of rotationally-supported disk galaxy models has been studied extensively since its first discovery over half a century ago. We were therefore very surprised to find cases of disks embedded in rigid halos, which on the basis of widely-cited criteria should be unstable, that appeared to be robustly stable. Here we show that the unstable bar mode in such simulations was being suppressed by changes to the disk caused by other instabilities having higher angular symmetry that were the first to saturate. Although this may seem like a promising solution to the long-standing puzzle presented by the apparent stability of real disk galaxies, we also show that instability is restored in the same models when the rigid halo is replaced by a live population of particles, where the usual stability conditions apply. Our study has been confined to a narrow range of models, and we cannot therefore exclude the possibility that mode interference may be able to prevent bar formation in other models having live halos.