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Constraints on the Magellanic Clouds' Interaction from the Distribution of OB Stars and the Kinematics of Giants

Young, OB-type candidates are identified in a ~7900 sq-deg. region encompassing the Large and Small Magellanic Clouds (LMC/SMC) periphery, the Bridge, part of the Magellanic Stream (MS) and Leading Arm (LA). Selection is based on UV, optical and IR photometry from existing large-area surveys and proper motions from the Southern Proper Motion 4 catalog (SPM4). The spatial distribution of these young star candidates shows: 1) a well-populated SMC wing which continues westward with two branches partially surrounding the SMC, 2) a rather narrow path from the SMC wing eastward toward the LMC which is offset by 1 to 2 deg. from the high-density H I ridge in the Bridge, 3) a well-populated periphery of the LMC dominated by clumps of stars at the ends of the LMC bar and 4) a few scattered candidates in the MS and two overdensities in the LA regions above and below the Galactic plane. Additionally, a proper-motion analysis is made of a radial-velocity selected sample of red giants and supergiants in the LMC, previously shown to be a kinematically and chemically distinct subgroup, most likely captured from the SMC. SPM4 proper motions of these stars also indicate they are distinct from the LMC population. The observational results presented here, combined with the known orbits of the Clouds, and other aspects of the LMC morphology, suggest an off-center, moderate to highly-inclined collision between the SMC and the LMC's disk that took place between 100 and 200 Myr ago.