Ripples spreading across the Galactic disc: Interplay of direct and indirect effects of the Sagittarius dwarf impact (2501.12436v1)

Abstract: Gaia data have revealed vertically asymmetric phase-space structures in the Milky Way (MW) disc, such as phase spirals, indicating vertical oscillations. These oscillations exhibit two distinct modes: the bending mode and the breathing mode, associated with one-arm and two-arm phase spirals, respectively. This study aims to explore the excitation mechanisms of the bending and breathing modes and their subsequent evolution in the MW disc, focusing on the interplay between direct perturbations from the Sagittarius dwarf galaxy and indirect contributions from tidally induced spiral arms. We perform high-resolution $N$-body simulations to model the interaction between an MW-like disc galaxy and a Sagittarius dwarf-like satellite. These simulations resolve fine phase-space structures, enabling analysis of the bending and breathing modes at both macroscopic (global bending and breathing waves) and microscopic (local phase spirals) scales. Our simulations demonstrate that the satellite's perturbation directly excites the bending mode and induces spiral arms in the galactic disc. These spiral arms excite the breathing mode, making it an indirect consequence of the satellite interaction. Initially, the bending mode dominates, but it rapidly decays due to horizontal mixing. In contrast, the breathing mode persists for a longer duration, sustained by the spiral arms, leading to a transition from a bending-dominated to a breathing-dominated state. This transition progresses faster in the inner galaxy than in the outer regions. The simulations reproduce the one-arm phase spiral observed in the solar neighbourhood and reveal two-arm phase spirals, particularly in the inner galaxy, associated with spiral arm-induced breathing modes. Our findings highlight the combined effects of direct satellite perturbations and indirect spiral arm dynamics in shaping the vertical structure of the MW disc.

• The paper uses five billion particle N-body simulations to study how the Sagittarius dwarf galaxy's impact causes vertical oscillations and phase-space spirals in the Milky Way disc.
• Direct effects from the impact cause rapidly damped bending modes, while indirect effects from resulting spiral arms excite persistent breathing modes, leading to a transition over time.
• These findings explain observed galactic structures like one and two-arm phase spirals and suggest a significant Sgr interaction occurred over 400 million years ago.

Analysis of "Ripples spreading across the Galactic disc"

The paper conducted by Asano et al. explores the dynamical repercussions of the Sagittarius (Sgr) dwarf galaxy's impact on the Milky Way (MW) disc, observed through the lens of vertically asymmetric structures known as phase-space spirals. The investigation employed $N$-body simulations with five billion particles to explore and decipher the genesis and evolution of vertical oscillations in the MW disc influenced by Sgr-like satellite perturbations.

Findings and Methodology

The research outlines two principal oscillation mechanisms in the MW disc: the bending mode and the breathing mode, each associated with distinct phase spirals. Using high-resolution simulations, the paper partitions the excitation process into two core types of effects:

1. Direct Effects: The Sgr impact directly provokes the bending mode, manifesting initially as dominant and characterized by rapid damping due to horizontal mixing. This mode is marked by noticeable bending waves across the MW disc.
2. Indirect Effects: The satellite-triggered formation of spiral arms within the MW disc subsequently excites the breathing mode. Unlike the bending mode, the breathing mode showcases durability and persistence owing to its excitation by these dynamically stable spiral arms. Over time, the disc transitions from a bending-dominated to a breathing-dominated state, progressing faster in the inner regions than the outer segments.

Additionally, the simulations reveal the emergence of one-arm phase spirals observable in the solar neighborhood and two-arm spirals prominently in the inner galaxy. The latter appear approximately 200-250 Myr following the transition from the bending-dominated phase.

Implications and Interpretations

The implications of these findings are manifold. The paper not only elucidates the layered complexities within the MW disc but also underscores the interactive dynamics between external perturbative influences and internal structural propagation. These insights highlight the necessity to consider both direct and indirect excitation mechanisms when exploring the MW's vertical dynamics.

Moreover, the results propose a historical perturbation impact by the Sgr, positing a significant interaction event over 400 Myr ago. This aligns with observed dynamical phenomena in the MW, providing a cohesive framework for understanding the underlying physics of galactic structures.

Future Directions

This research lays the groundwork for future explorations into the ramifications of satellite impacts on galactic discs. Potential areas of progression include enhancements in simulation resolution and scope, allowing for a broader investigation into the interactions between multiple satellite systems and host galaxies. Furthermore, comparative studies that align observational data from missions such as Gaia with simulation outputs can refine our understanding of the MW's dynamic evolution and structure-forming processes at play.

In conclusion, this paper exemplifies a rigorous approach to unraveling the complex interaction between a host galaxy and its perturbative satellites, offering crucial insights into the vertical dynamics and historical perturbations of the MW disc.