Research Byte

Published in the RSAA Lunations
Vol1 Issue18 1–31 July 2021

As the closest pair of interacting dwarf galaxies to us, the Large and Small Magellanic Clouds (LMC/SMC) constitute the prototype system for studying the influence of tidal interactions on galaxy evolution. In order to trace the complex interaction history between both Clouds and the Milky Way, we must study the highly-substructured outskirts of the Clouds, where dynamical timescales are long, and the resulting structural and kinematic signatures are persistent.  

To kinematically study these structures, the Magellanic Edges Survey (MagES) is an ideal tool. It is a spectroscopic study using the 2dF+AAOmega instrument on the Anglo-Australian Telescope, targeting ~8700 red clump and red giant branch stars across the periphery of the Clouds. In combination with astrometric measurements and high-precision photometry from the Gaia satellite, the survey provides 3D kinematics and abundance information critical for understanding the effects of dynamical perturbations on the Clouds.  

The MagES team recently investigated the origin of a >20 degree long arm-like feature in the northern outskirts of the LMC. We study the metallicity, structure, and kinematics of the arm, finding it has a similar geometry and [Fe/H] abundance to the nearby outer LMC disk, and is likely comprised of perturbed LMC disk material. Whilst the azimuthal (rotation) 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.  

Comparison with a new suite of dynamical models of the Magellanic/Milky Way system reveals the tidal force of the Milky Way during the LMC’s infall is primarily responsible for out-of-plane perturbations along the arm. We also find that close LMC/SMC interactions within the past Gyr -- particularly the SMC’s pericentric passage ~150 Myr ago and a recent SMC crossing of the LMC disk plane ~400 Myr ago -- likely do not perturb stars that today comprise the arm, and are instead likely responsible for structures in the western LMC disk. However, 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 a strongly negative in-plane radial velocity which current models cannot reproduce. 

Interested readers can find the associated paper at https://arxiv.org/abs/2106.03274 

Lara Cullinane