Title: The motions of the Magellanic Clouds and the nature of galactic dark matter
Description: I present a measurement of the systemic proper motions of the Large & Small Magellanic Clouds (LMC & SMC) made using the Advanced Camera for Surveys on the Hubble Space Telescope . We tracked the LMC's(SMC's) motion relative to 21(5) background QSOs over a baseline of approximately 2 years. The measured proper motion for the LMC is accurate to better than 5%: μ W = -2.03 ± 0.08 mas yr -1 , μ N = 0.44 ± 0.05 mas yr -1 , while that for the SMC is accurate to 15%: μ W = -1.16 ± 0.18 mas yr -1 , μ N = -1.17 ± 0.18 mas yr -1 . This is a 7(4)-fold improvement over previous measurements for the LMC(SMC), and is sufficient to provide improved constraints on the properties of the Milky Way (MW) dark halo. The current relative velocity between the Clouds, 105 ± 42 km s -1 , is substantially higher than that used in theoretical models of the Magellanic Stream. Investigations of the past orbital motions of the Clouds in an isothermal model for the dark halo of the MW imply that the Clouds could be coincidental companions, unbound from each other. However, bound orbits are also found within our error circles. Smaller proper motion errors and better understanding of L/SMC masses would be required to constrain their bound vs. unbound nature unambiguously. The absolute velocities of the Clouds are also substantially higher (∼ 100 km s -1 ) than those used in models. Consequently, in a ΛCDM-motivated MW model ( M vir = 10 12 [Special characters omitted.] ), we find that the Clouds are likely only on their first passage, which would require a major reassessment of formation mechanisms for the Magellanic Stream. Alternatively, the MW dark halo must be more massive than presently believed, M vir > 2 × 10 12 [Special characters omitted.] , which may conflict with other observational constraints. I also investigate MW structure via microlensing.

We have used the Infrared Array Camera (IRAC) on the Spitzer Space Telescope to obtain near/mid-infrared imaging for 15 LMC microlensing events seen in the original MACHO survey to characterize the lensing population. This has resulted in the identification of two MACHO lenses, via their infrared excesses, as M dwarfs in the MW disk. These are the only two microlenses identified to date. Together, they are still consistent with the expected frequency of nearby stars in the MW thin and thick disks acting as lenses. I discuss the region of phase-space in terms of distance and magnitude that can be probed with IRAC, and hence what the results mean for the microlensing population in general.