Effects of the Central Mass Concentration on Bar Formation in Disk Galaxies

TL;DR

This study uses N-body simulations to explore how the central mass concentration influences bar formation in disk galaxies, revealing specific stability conditions and the impact of bulge and halo properties on bar strength and rotation.

Contribution

It introduces a new stability criterion involving Toomre's Q and central mass concentration, linking galaxy structure to bar formation outcomes.

Findings

Bars are stronger and longer in galaxies with less massive bulges and halos.

All simulated bars are slow rotators, matching observations of the Milky Way.

A specific bulge-to-disk mass ratio under a concentrated halo produces Milky Way-like bars.

Abstract

While bars are common in disk galaxies, their formation conditions are not well understood. We use $N$-body simulations to study bar formation and evolution in isolated galaxies consisting of a stellar disk, a classical bulge, and a dark halo. We consider 24 galaxy models that are similar to the Milky Way but differ in the mass and compactness of the classical bulge and halo concentration. We find that the bar formation requires $(Q_{T,\text{min}}/1.2)^2 + (\text{CMC}/0.05)^2\lesssim 1$, where $Q_{T,\text{min}}$ and CMC refers to the minimum value of the Toomre stability parameter and the central mass concentration, respectively. Bars tend to be stronger, longer, and rotate slower in galaxies with a less massive and less compact bulge and halo. All bars formed in our models correspond to slow bars. A model with the bulge mass of $\sim10$--$20$\% of the disk under a concentrated halo produces a bar similar to the Milky-Way bar. We discuss our findings in relation to other bar formation criteria suggested by previous studies.