Proper Motion of the Magellanic Clouds and the Dynamics of the Local Group

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Session 1 -- Invited Talk
Oral presentation, Monday, 8:30-9:20, Zellerbach Auditorium Room

[1.01] Proper Motion of the Magellanic Clouds and the Dynamics of the Local Group

D.N.C. Lin (Lick Observatory, University of California, Santa Cruz.)

We have measured the proper motion of the Large Magellanic Cloud (LMC) using 21 plates taken with the CTIO 4 m telescope covering an epoch span of 14 years. The plates were centered on the globular cluster NGC 2257, lying on the north-east periphery of the cloud. Proper motions were determined for two hundred fifty one LMC members, with respect to 92 galaxies. After correcting for the rotation of the LMC and the effects of solar motion, this imply a galacto-centric coordinate radial velocity for the cloud of 54 km s$^{-1}$ and a total galacto-centric transverse velocity of 236 km s$^{-1}$. The direction of LMC's motion is leading the Magellanic Stream which is consistent with the scenario that the H I gas in the Magellanic Stream was originally bound to the Magellanic Clouds and was tidally torn from them by the Galaxy within the past 14 billion years. From the large magnitude of the Magellanic Clouds' proper motion and that of the radial velocities in the Magellanic Stream we infer 1) the Magellanic Clouds are near their perigalacticon, 2) they are gravitationally bound to the Galaxy with apogalacticon distance well beyond 100 kpc, and 3) they carry comparable amount of total angular momentum as the Population I stars in the Galactic disk. We also determine that 1) the Galactic halo has a total mass $\sim 6 \pm 2 \times 10^{11} M_\odot$ within 100 kpc, 2) a substantial fraction of this mass is distributed beyond the present Galactic distance of the Magelannic Clouds ($\ge 50$ kpc), 3) the gravitational potential of the Galactic halo is essentially spherically symmetric, and 4) most of the present mass interior to the $\sim 50$ kpc was essentially acquired by the Galaxy when the Magellanic Clouds become bound satellites of the Galaxy. We also show that there is reasonable possibility that 1) the Small and Large Magellanic Clouds were bound binary galaxies, 2) a number of high velocity and young globular clusters were tidally torn from the Magellanic Clouds during previous perigalacticon passages, and 3) some of the dwarf galaxies may also be tidal debris of the Magellanic Clouds.

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