Excitation of vertical breathing motion in disc galaxies by tidally-induced spirals in fly-by interactions (2208.07096v1)

Abstract: It is now clear that the stars in the Solar neighbourhood display large-scale coherent vertical breathing motions. At the same time, Milky Way-like galaxies experience tidal interactions with satellites/companions during their evolution. While these tidal interactions can excite vertical oscillations, it is still not clear whether vertical breathing motions are excited \textit{directly} by the tidal encounters or are driven by the tidally-induced spirals. We test whether excitation of breathing motions are directly linked to tidal interactions by constructing a set of $N$-body models (with mass ratio 5:1) of unbound, single fly-by interactions with varying orbital configurations. We first reproduce the well-known result that such fly-by interactions can excite strong transient spirals (lasting for $\sim 2.9-4.2$ Gyr) in the outer disc of the host galaxy. The generation and strength of the spirals are shown to vary with the orbital parameters (the angle of interaction, and the orbital spin vector). Furthermore, we demonstrate that our fly-by models exhibit coherent breathing motions whose amplitude increases with height. The amplitudes of breathing motions show characteristic modulation along the azimuthal direction, with compressing breathing motions coinciding with the peaks of the spirals and expanding breathing motions falling in the inter-arm regions -- a signature of a spiral-driven breathing motion. These breathing motions in our models end when the strong tidally-induced spiral arms fade away. Thus, it is the tidally-induced spirals which drive the large-scale breathing motions in our fly-by models, and the dynamical role of the tidal interaction in this context is indirect.

• The paper investigates how tidally-induced spiral arms trigger vertical breathing motions in disc galaxies using detailed N-body simulations.
• It demonstrates a two-phase winding process with maximum spiral strength beyond 10 kpc and lifetimes ranging from 2.9 to 4.2 Gyr.
• Findings reveal that spiral-driven compressive and expansive motions modulate stellar movement across the galactic mid-plane, influencing galaxy evolution.

Excitation of Vertical Breathing Motion in Disc Galaxies by Tidally-Induced Spirals

In the field of galactic dynamics, understanding the vertical oscillations within disc galaxies serves as a pivotal consideration. The paper "Excitation of vertical breathing motion in disc galaxies by tidally-induced spirals in fly-by interactions" by Kumar et al., explores this topic by examining the effects of tidal interactions on disc galaxies. Here, tidally-induced spirals are analyzed to assess their role in driving large-scale breathing motions.

Methodology and Findings

Using a sophisticated set of N-body simulations with a fixed mass ratio of 5:1, the paper constructs various fly-by interaction scenarios to explore the resulting dynamical phenomena. The simulations meticulously alter orbital configurations, thereby permitting an examination of spirals' genesis, strength, and longevity as influenced by the orbital parameters.

A salient feature observed is that a fly-by interaction triggers the formation of spiral arms in the outer regions of the host galaxy. The results indicate that these spirals are density waves rather than material arms, consistent with classical density wave theory. The spirals initiated by tidal interactions exhibit a maximum strength within a defined radial range beyond 10 kpc, and they are characterized by their transient nature, lasting about 2.9 to 4.2 Gyr. Such spirals demonstrate an intriguing two-phase winding process: an initial rapid winding, succeeded by a slower winding phase.

A significant contribution of this research lies in the elucidation of breathing motions—where stars on both sides of the galactic mid-plane move towards or away from it. Notably, the presence of spirals is directly associated with coherent breathing motions. These motions reveal a clear azimuthal modulation: compressive motions align with spiral peaks and expansive motions occur in inter-arm regions. This alignment signifies that spirals, rather than the tidal interaction per se, are crucial in driving such vertical motions.

Implications and Future Directions

The implications of this paper extend to multiple facets of galactic dynamics and ultimate galaxy evolution. Understanding the excitation and dissipation of breathing motions sheds light on the energy distribution and vertical wave propagation within galactic discs, important for the structural stability of galaxies.

Furthermore, this paper enhances theoretical frameworks relating to the interplay between internal and external factors affecting galaxies. The findings assert that the role of spiral arms, even when induced by external influences, is profound in shaping the evolution of galaxy kinematics.

Future research could further explore the interplay between gas dynamics and stellar motion, as the present paper is limited to N-body simulations. Additionally, addressing varying mass ratios and more complex interaction scenarios can enrich the understanding of galactic disc evolution in a cosmological context. The role of additional galactic features such as bars and bulges in modulating these motions presents another avenue for exploration.

In summary, this paper provides a detailed insight into the complex mechanisms through which fly-by interactions induce spiral structures and their subsequent impact on galaxy dynamics, particularly focusing on breathing motions, thereby contributing to a deeper understanding of disc galaxy evolution.