Migration and Mixing in the Galactic Disc from Encounters between Sagittarius and the Milky Way

TL;DR

This study investigates how the Sagittarius dwarf galaxy influences stellar migration in the Milky Way's disc, revealing observable signatures like metallicity variations and orbital eccentricity changes caused by external interactions.

Contribution

It extends migration studies by analyzing external tidal effects from Sagittarius using impulse approximation and N-body simulations, identifying unique signatures of such interactions.

Findings

Sagittarius tidal passages cause quadrupole-like radial migration patterns.

Migration signatures include azimuthal metallicity variations and eccentricity changes.

External influences can dominate over secular evolution in the outer disc.

Abstract

Stars born on near-circular orbits in spiral galaxies can subsequently migrate to different orbits due to interactions with non-axisymmetric disturbances within the disc such as bars or spiral arms. This paper extends the study of migration to examine the role of external influences using the example of the interaction of the Sagittarius dwarf galaxy (Sgr) with the Milky Way (MW). We first make impulse approximation estimates to characterize the influence of Sgr disc passages. The tidal forcing from Sgr can produce changes in both guiding radius ($\Delta R_g$) and orbital eccentricity, as quantified by the maximum radial excursion, $\Delta R_ {\rm max} $. These changes follow a quadrupole-like pattern across the face of the disc, with amplitude increasing with Galactocentric radius. We next examine a collisionless N-body simulation of a Sgr-like satellite interacting with a MW-like galaxy and find that Sgr's influence in the outer disc dominates over the secular evolution of orbits between disc passages. Finally, we use the same simulation to explore possible observable signatures of Sgr-induced migration by painting the simulation with different age stellar populations. We find that following Sgr disc passages, the migration it induces manifests within an annulus as an approximate quadrupole in azimuthal metallicity variations ($\delta_ {\rm [Fe/H]} $), along with systematic variations in orbital eccentricity, $\Delta R_ {\rm max} $. These systematic variations can persist for several rotational periods. We conclude that this combination of signatures may be used to distinguish between the different migration mechanisms shaping the chemical abundance patterns of the Milky Way's thin disc.