The orbital dynamics of the LMC and SMC about the Milky Way

This review summarizes the changing observational and theoretical landscape that has led to rapid developments in our understanding of the orbital history of the Milky Way's most massive satellite galaxies, the LMC and SMC. The determination of high precision center-of-mass proper motions for the Clouds and their environs, coupled with new insights into the structure of dark matter halos from cosmological simulations, indicates that the Clouds are recent additions to the Milky Way's satellite system, where the LMC was sufficiently massive at infall to induce distortions in the Milky Way's dark matter halo, and bring in satellites of its own. To gain insights into the orbital history of the Clouds it is, therefore, necessary to integrate orbits in a time-evolving Milky Way + LMC potential. The proper motion measurements further indicate that the LMC and SMC have had a recent strong encounter with each other. This leaves open the impact of this encounter on their internal structures, and the degree of correspondence between their stellar, HI and dark matter centers. The choice of kinematic center in turn affects the center of mass proper motions; this is the biggest observational unknown, even in the high precision Gaia era. Given the expected mass loss of the SMC, the distortions it induces in the LMC halo, and the recent LMC-SMC close encounter, understanding the orbits of objects about the LMC (including the SMC) and interpreting observational data of the Clouds requires full N-body simulations that are constrained by their present-day structure and kinematics.