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N. Kallivayalil (CfA), R.P. van der Marel (STScI), C. Alcock (CfA), T. Axelrod (Steward Obs.), K.H. Cook (LLNL), A.J. Drake (Caltech), M. Geha (Carnegie Obs.)
The interaction between the Large and Small Magellanic Clouds (LMC & SMC) and the Milky Way (MW) is thought to have played an important role in the dynamical evolution of the MW's outer parts. The Clouds probe the potential of the MW dark halo in places where other kinematic tracers are unavailable and thus the MW-MC system has been a major subject of study. In particular, the global dynamics of both Clouds need to be well prescribed before other evolutionary features of the system can be understood. The radial velocities of the clouds are more readily determined than the transverse velocities, which can only be estimated using proper motions.
We undertook a project using two epochs of HST/ACS data to determine the systemic proper motions of the Clouds. The Magellanic Cloud fields are centered on background QSOs that were discovered from their optical variability in the MACHO database (Geha et al. 2003). The final sample consists of 21 QSOs behind the LMC and 5 behind the SMC, distributed homogeneously behind the central few degrees of both Clouds. With a ~ year baseline and the use of the High Resolution Camera, we have determined the proper motion of the LMC to better than 5 \ \muN = 0.44 ±0.05 mas/yr (Kallivayalil et al. 2005). This is the most accurate proper motion measurement for any MW satellite thus far. We will present this measurement, as well as our results for the SMC, and the conclusions we can draw about the Clouds' orbits around the MW.
Our study shows that ground-based work on finding QSOs can be combined with high resolution HST data to get good measurements in a relatively short amount of time. When combined with HI data from the Magellanic Stream our measurements should provide new constraints on both the mass distribution of the Galactic Halo and models of the Stream.
Support for this work was provided by NASA through grant numbers GO-09462 and GO-10130 from STScI. KHC's work was performed under the auspices of the U.S. DOE, NNSA, by the Univ. of California, LLNL under contract No. W-7405-Eng-48.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.