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

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.

• The paper demonstrates that minor galaxy flybys induce significant disk thickening by increasing the disk scale height to radius ratio.
• The paper shows that tidal interactions trigger the formation of pronounced spiral arms, especially in pseudobulge-hosting galaxies with pre-existing bars.
• The paper finds that classical bulges remain morphologically stable during flybys, while pseudobulges experience measurable angular momentum changes.

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

The paper "Galaxy Flybys: Evolution of the Bulge, Disk, and Spiral Arms" investigates the impact of minor galaxy flybys on the structural components of Milky Way-mass galaxies. Specifically, it analyzes the bulge, disk, and spiral arms with a focus on different bulge morphologies—classical bulges and boxy/peanut pseudobulges. The study employs N-body simulations to model interactions between disk galaxies of different mass ratios (10:1 and 5:1) while varying the pericenter distances of their orbits.

Methodology

Simulations are based on galaxies with varied bulge types, demonstrating the interactions between major and minor galaxies. The classical bulge model is derived from a Hernquist density distribution, while the pseudobulge grows from a buckling bar in pre-evolved galaxies. Tools such as GALFIT were utilized for simulated two-dimensional image decomposition to study photometric and kinematic parameters over simulation time. Additionally, Fourier analysis methods were employed to quantify spiral arm strength and characterize disk perturbations during flybys.

Key Findings

1. Disk Evolution: Flybys primarily induce disk thickening, manifested as an increase in the ratio of disk scale height to scale radius. While absolute height changes are minimal, the relative scale height to radius ratios show significant variation, particularly in close flybys and with greater satellite mass ratios.
2. Spiral Arms: Tidal interactions effectively stimulate the formation of pronounced spiral arms. The strength of these structures saw oscillations in pseudobulge-hosting galaxies, attributed to pre-existing bar structures. Strength peaked immediately after pericenter passage and normalized over time.
3. Bulge Stability: Classical bulges are notably resilient to flyby-induced morphological and dynamical changes. The study illustrates that classical bulges maintain their shape and angular momentum, even when exposed to significant gravitational interactions. Observations of bulge properties suggest a pseudo-transformation in face-on views, while the actual bulge morphology remains unchanged.
4. Angular Momentum Transfer: The research highlights minimal angular momentum acquisition by classical bulges, primarily sourced from underlying rotating disk material. In contrast, pseudobulges—integral to a galaxy's bar—demonstrate notable angular momentum increases, partially accentuating the bar's dynamic evolution through flybys.
5. Bar Buckling: Interestingly, flybys had negligible influence on both the timing and intensity of bar buckling. The research suggests that existing bars, and by extension pseudobulges formed from such bars, resist significant perturbation by flybys with minor galaxies.

Implications and Future Research

The study contributes critical insights into galaxy evolution dynamics, challenging the significance of classical bulge transformation through minor interactions. It reaffirms the stability of classical bulges in gravitational interactions, suggesting any apparent transformations may be the result of disk-complex interactions during photometric analysis rather than actual morphological changes.

Future investigations may benefit from including gas dynamics to assess the compounded effects of star formation and feedback processes during galactic flybys. Additionally, the impacts of successive flyby events or interactions with major galaxies could offer a more comprehensive understanding of galactic morphological evolutions over cosmic time. Future work aiming to include more complex models with gas dynamics could help in understanding star formation rates and their scaling with flyby-induced morphological changes.

This paper underscores the nuanced and varied influence of flybys on galaxy morphology, reinforcing the resilience of certain structural components while elucidating the conditions under which dynamic changes in galaxy formation and evolution might occur.