  Recent observational and theoretical studies on the three-dimensional (3D)
space motions of the Large and the Small Magellanic Clouds (LMC and SMC,
respectively) have strongly suggested that the latest proper motion
measurements of the Magellanic Clouds (MCs) are consistent with their 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 the mass larger
than ~ 2 * 10^{10} M_sun, 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.
