Bar instability in disk-halo systems

TL;DR

This paper demonstrates that disk-halo systems with live, responsive halos are more prone to bar instabilities than rigid halos, with the shape of the halo's velocity ellipsoid influencing the instability's strength.

Contribution

It reveals that stability criteria for disks in rigid halos do not hold for live, responsive halos and explores how halo shape affects bar formation.

Findings

Live halos increase bar growth rates compared to rigid halos.

Halo shape influences the vigor of bar instability.

Rigid halo stability criteria are not applicable to live halos.

Abstract

We show that the exponential growth rate of a bar in a stellar disk is substantially greater when the disk is embedded in a live halo than in a rigid one having the same mass distribution. We also find that the vigor of the instability in disk-halo systems varies with the shape of the halo velocity ellipsoid. Disks in rigid halos that are massive enough to be stable by the usual criteria, quickly form bars in isotropic halos and much greater halo mass is needed to avoid a strong bar; thus stability criteria derived for disks in rigid halos do not apply when the halo is responsive. The study presented here is of an idealized family of models with near uniform central rotation and that lack an extended halo; we present more realistic models with extended halos in a companion paper. The puzzle presented by the absence of strong bars in some galaxies having gently rising inner rotation curves is compounded by the results presented here.