2008ApJ...684L..87B

Recent observational and theoretical studies on the three-dimensional (3D) space motions of the Large and Small Magellanic Clouds (LMC and SMC, respectively) have strongly suggested that the latest proper-motion measurements of the Magellanic Clouds (MCs) are consistent with orbital evolution models in which the MCs have arrived in the Galaxy quite recently for the first time. The suggested orbital models appear to be seriously inconsistent with the tidal interaction models in which the Magellanic Stream (MS) can be formed as a result of the mutual tidal interaction between the MCs and the Galaxy for the last ∼2 Gyr. Based on orbital models of the MCs, we propose that if the MCs have a common diffuse dark halo with a mass larger than ∼2x10¹⁰ M_☉, the MCs can not only have the present 3D velocities consistent with the latest proper-motion measurements but also interact strongly with each other and with the Galaxy for the last 2 Gyr. These results imply that if the observed proper motions of the MCs are true ones of the centers of mass for the MCs, the common halo of the MCs would need to be considered in constructing self-consistent MS formation models. We discuss whether the origin of the possible common halo can be closely associated either with the past binary formation or with the MCs having been in a small group.