The Magellanic Edges Survey -- II. Formation of the LMC's northern arm

TL;DR

This study investigates the origin of a large arm-like feature in the northern outskirts of the LMC, revealing it is composed of perturbed disk material influenced by the Milky Way's tidal forces and past interactions with the SMC.

Contribution

The paper combines observational data and dynamical modeling to elucidate the formation of the LMC's northern arm, highlighting the role of the Milky Way's tidal forces and historical SMC interactions.

Findings

The arm has similar geometry and metallicity to the outer LMC disk.

Kinematic perturbations suggest influence from the Milky Way's tidal forces.

Past SMC interactions may explain the arm's unique velocity properties.

Abstract

The highly-substructured outskirts of the Magellanic Clouds provide ideal locations for studying the complex interaction history between both Clouds and the Milky Way (MW). In this paper, we investigate the origin of a >20$^\circ$ long arm-like feature in the northern outskirts of the Large Magellanic Cloud (LMC) using data from the Magellanic Edges Survey (MagES) and Gaia EDR3. We find that the arm has a similar geometry and metallicity to the nearby outer LMC disk, indicating that it is comprised of perturbed disk material. Whilst the azimuthal velocity and velocity dispersions along the arm are consistent with those in the outer LMC, the in-plane radial velocity and out-of-plane vertical velocity are significantly perturbed from equilibrium disk kinematics. We compare these observations to a new suite of dynamical models of the Magellanic/MW system, which describe the LMC as a collection of tracer particles within a rigid potential, and the SMC as a rigid Hernquist potential. Our models indicate the tidal force of the MW during the LMC's infall is likely responsible for the observed increasing out-of-plane velocity along the arm. Our models also suggest close LMC/SMC interactions within the past Gyr, particularly the SMC's pericentric passage ~150 Myr ago and a possible SMC crossing of the LMC disk plane ~400 Myr ago, likely do not perturb stars that today comprise the arm. Historical interactions with the SMC prior to ~1 Gyr ago may be required to explain some of the observed kinematic properties of the arm, in particular its strongly negative in-plane radial velocity.