A Study of the Effect of Bulges on Bar Formation in Disk galaxies

TL;DR

This study uses N-body simulations to investigate how bulge mass and concentration influence bar formation in disk galaxies, revealing a critical bulge-to-disk force ratio that inhibits bar development.

Contribution

It introduces a new criterion based on bulge-to-disk force ratio for predicting bar formation, expanding understanding of bulge effects beyond halo influences.

Findings

Bars cannot form if bulge-to-disk force ratio exceeds 0.35.

Denser bulges have a lower mass ratio cutoff for bar suppression.

Early type galaxies can still develop strong bars despite massive bulges.

Abstract

We use N-body simulations of bar formation in isolated galaxies to study the effect of bulge mass and bulge concentration on bar formation. Bars are global disk instabilities that evolve by transferring angular momentum from the inner to outer disks and to the dark matter halo. It is well known that a massive spherical component such as halo in a disk galaxy can make it bar stable. In this study we explore the effect of another spherical component, the bulge, on bar formation in disk galaxies. In our models we vary both the bulge mass and concentration. We have used two sets of models, one that has a dense bulge and high surface density disk. The second model has a less concentrated bulge and a lighter disk. In both models we vary the bulge to disk mass fraction from 0 to 0.7. Simulations of both the models show that there is an upper cutoff in bulge to disk mass ratio M b /M d above which bars cannot form; the cutoff is smaller for denser bulges( M b /M d = 0.2) compared to less denser ones (M b /M d = 0.5). We define a new criteria for bar formation in terms of bulge to disk radial force ratio (F b /F d ) at the disk scale lengths above which bars cannot form. We find that if F b /F d > 0.35, a disk is stable and a bar cannot form. Our results indicate that early type disk galaxies can still form strong bars in spite of having massive bulges.