Galaxy Flybys: Evolution of the Bulge, Disk, and Spiral Arms

TL;DR

This study uses N-body simulations to analyze how minor galaxy flybys influence the structural and dynamical evolution of bulges, disks, and spiral arms in Milky Way-like galaxies, highlighting effects on pseudobulges and disk thickening.

Contribution

It provides new insights into the effects of minor flybys on different bulge types and disk structures, emphasizing the stability of classical bulges and the dynamical heating of pseudobulges.

Findings

Flybys cause disk thickening and minor spiral structure oscillations.

Classical bulges remain unaffected and stable during flybys.

Pseudobulges become dynamically hotter, indicating accelerated secular evolution.

Abstract

Galaxy flybys are as common as mergers in low redshift universe and are important for galaxy evolution as they involve the exchange of significant amounts of mass and energy. In this study we investigate the effect of minor flybys on the bulges, disks, and spiral arms of Milky Way mass galaxies for two types of bulges - classical bulges and boxy/peanut pseudobulges. Our N-body simulations comprise of two disk galaxies of mass ratios 10:1 and 5:1, where the disks of the galaxies lie in their orbital plane and the pericenter distance is varied. We performed photometric and kinematic bulge-disk decomposition at regular time steps and traced the evolution of the disk size, spiral structure, bulge sersic index, bulge mass, and bulge angular momentum. Our results show that the main effect on the disks is disk thickening, which is seen as the increase in the ratio of disk scale height to scale radius. The strength of the spiral structure A2/A0 shows small oscillations about the mean time-varying amplitude in the pseudobulge host galaxies. The flyby has no significant effect on non-rotating classical bulge, which shows that these bulges are extremely stable in galaxy interactions. However, the pseudobulges become dynamically hotter in flybys indicating that flybys may play an important role in accelerating the rate of secular evolution in disk galaxies. This effect on pseudobulges is a result of their rotating nature as part of the bar. Also, flybys do not affect the time and strength of bar buckling.