Why are some galaxies not barred?

TL;DR

This study uses collisionless simulations to explore why some galaxies do not develop bars, finding that compact classical bulges can prevent bar formation for over 4 billion years by disrupting typical bar growth mechanisms.

Contribution

It demonstrates that dense classical bulges can inhibit bar formation in stellar disks, providing a new explanation for un-barred galaxies.

Findings

Compact bulges prevent bar formation for at least 4 Gyr.

High-density bulges disrupt inner Lindblad resonances.

Rapid central heating correlates with bar suppression.

Abstract

Although more than two-thirds of star-forming disk galaxies in the local universe are barred, some galaxies remain un-barred, occupying the upper half of the Hubble tuning fork diagram. Numerical simulations almost always produce bars spontaneously, so it remains a challenge to understand how galaxies sometimes prevent bars from forming. Using a set of collisionless simulations, we first reproduce the common result that cold stellar disks surrounding a classical bulge become strongly unstable to non-axisymmetric perturbations, leading to the rapid formation of spiral structure and bars. However, our analyses show that galaxy models with compact classical bulges (whose average density is greater than or comparable to the disk density calculated within bulge half-mass radii) are able to prevent bar formation for at least 4 Gyr even when the stellar disk is maximal and having low Toomre Q. Such bar prevention is the result of several factors such as (a) a small inner Lindblad resonance with a high angular rate, which contaminates an incipient bar with $x_2$ orbits, (b) rapid loss of angular momentum accompanied by a rapid heating in the center from initially strong bar and spiral instabilities in a low-Q disk, in other words, a rapid initial rise to a value larger than $\sim5$ of the ratio of the random energy to the rotational energy in the central region of the galaxy.