The stability of some galaxy disks is still perplexing

TL;DR

This paper investigates why some galaxy disks avoid bar formation despite inherent instabilities, testing various stability strategies through simulations, and finding that common approaches like central heating or mass tapering are ineffective.

Contribution

The study systematically tests the effects of central dynamical heating and mass tapering on galaxy disk stability, revealing their limited impact on preventing bar formation.

Findings

Central heating and mass tapering do not significantly enhance disk stability.

Deep density cutouts prevent feedback through the center but still lead to bar instabilities.

Simulations show that common stability strategies are ineffective against bar formation.

Abstract

The problem of how some disk galaxies avoid forming bars remains unsolved. Many galaxy models having reasonable properties continue to manifest vigorous instabilities that rapidly form strong bars and no widely-accepted idea has yet been advanced to account for how some disk galaxies manage to avoid this instability. It is encouraging that not all galaxies formed in recent cosmological simulations possess bars, but the dynamical explanation for this result is unclear. The unstable mode that creates a bar is understood as a standing wave in a cavity that reflects off the disk center and the corotation radius, with amplification at corotation. Here we use simulations to address one further idea that may perhaps inhibit the feedback loop and therefore contribute to stability, which is to make the disk center dynamically hot and/or to taper away mass from the inner disk, which could be masked by a bulge. Unfortunately, we find that neither strategy makes much difference to the global stability of the disk in the models we have tried. While deep density cutouts do indeed prevent feedback through the center, they still reflect incoming waves and thereby provoke a slightly different instability that again leads to a strong bar.