The motion of the Magellanic clouds, origin of the Magellanic Stream, and the mass of the Milky Way

Abstract

Recent proper-motion observations indicate that the Magellanic Clouds are leading the Magellanic Stream. Although the direction of this motion is in agreement with that expected from theoretical models for the dynamics of the Clouds and the origin of the Stream, its magnitude is considerably smaller than that predicted. We reexamine the original conjecture that the Galaxy, during the past 14 x 10(9) yr, has tidally disrupted the LMC-SMC binary galaxy and stripped H I gas from the Clouds to form the Magellanic Stream. We adopt two general prescriptions for the Galactic potential and construct the past orbital history of several segments of the Stream. We show that the discrepancy between the observational data and previous theoretical predictions can be reconciled if, and only if, the Galactic halo has a mass similar to 5.5 +/- 1 x 10(11) M. within 100 kpc and a substantial fraction (similar to 1/2) of this mass is distributed beyond the present Galactic distance of the Magellanic Clouds (greater than or equal to 50 kpc). This mass is nearly half that assumed in the previous models, but it is consistent with some recent estimates for the Galactic halo. Beyond 100 kpc this mass may continue to increase to similar to 10(12) M. within its tidal radius (similar to 300 kpc). However, the modification of the Galactic potential does not lead to any major alteration in the dynamical evolution of the Magellanic Clouds. Similar to previous theoretical results, our analyses show that (1) the Magellanic Clouds are near perigalaction, (2) they are gravitationally bound to the Galaxy with apogalaction beyond 100 kpc, (3) the SMC and LMC are binary galaxies and will become separated in the next 1-2 Gyr, (4) the Clouds possess a total angular momentum comparable to that of Population I stars in the Galactic disk, (5) gas in the Stream is likely to have originated from the SMC between 1-2 Gyr ago; and (6) the mass in the Stream is comparable to the gas content in the SMC. Finally, we predict both the distance and proper motion of the gas and stars along the Stream, which may be measured to verify the validity of the theoretical model. We also predict the Galactocentric transverse velocity of the SMC to be 200 +/- 100 km s(-1).