Measuring the vertical response of the Galactic disc to an infalling satellite

TL;DR

This study uses N-body simulations to analyze how the Milky Way's disc responds vertically to impacts from a satellite galaxy, revealing the evolution of distortions and their potential to inform about galactic interactions.

Contribution

It provides a quantitative analysis of the vertical distortions in the Galactic disc caused by satellite impacts, using Fourier decomposition and demonstrating how to recover the response from observational data.

Findings

Vertical distortions can be modeled with Fourier terms m=0,1,2.

The m=0 term appears as ring-like distortions after impacts.

Vertical pattern speeds help constrain Milky Way-satellite interactions.

Abstract

Using N-body simulations of the Milky Way interacting with a satellite similar to the Sagittarius dwarf galaxy, we quantitatively analyse the vertical response of the Galactic disc to the satellite's repeated impacts. We approximate the vertical distortion of the Galactic disc as the sum of the first three Fourier azimuthal terms m = 0, 1 and 2, and observe their evolution in different dynamical regimes of interaction. After the first interaction, the m=0 term manifests itself as outgoing ring-like vertical distortions. The m=1 term (S-shape warp) is prograde when the impacts of the satellite are more frequent, or in general close to an interaction, whereas it is slowly retrograde in the most quiescent phases. The m=2 term is typically prograde, and close to an interaction it couples with the m=1 term. Finally, we find that the vertical response of the disc can be recovered in an unbiased way using the instantaneous positions and velocities of stars in a limited volume of the Galactic disc, analogous to real data, and that the measured vertical pattern speeds have a constraining power in the context of a Milky Way-satellite interaction.