Dynamics of the Magellanic Clouds in a LCDM Universe
  We examine Milky Way-Magellanic Cloud systems selected from the Millennium-II
Simulation in order to place the orbits of the Magellanic Clouds in a
cosmological context. Our analysis shows that satellites massive enough to be
LMC analogs are typically accreted at late times. Moreover, those that are
accreted at early times and survive to the present have orbital properties that
are discrepant with those observed for the LMC. The high velocity of the LMC,
coupled with the dearth of unbound orbits seen in the simulation, argues that
the mass of the MW's halo is unlikely to be less than 2 x 10^12 Msun. This
conclusion is further supported by statistics of halos hosting satellites with
masses, velocities, and separations comparable to those of the LMC. We further
show that: (1) LMC and SMC-mass objects are not particularly uncommon in
MW-mass halos; (2) the apparently high angular momentum of the LMC is not
cosmologically unusual; and (3) it is rare for a MW halo to host a LMC-SMC
binary system at z=0, but high speed binary pairs accreted at late times are
possible. Based on these results, we conclude that the LMC was accreted within
the past four Gyr and is currently making its first pericentric passage about
the MW.
