This investigation has made use of plate and CCD material of the Yale-San Juan Southern Proper Motion program, to obtain a catalog of absolute proper motions in an area of 450 square-degree that encloses the Magellanic Clouds and includes 1,448,438 objects. The main goal has been to obtain an independent measurement of the mean proper motion of the Large and Small Magellanic Clouds, in view of the most recent space-based proper motions which challenge the standard picture of the Clouds as a binary system of galaxies that travel in a bound orbit around the Galaxy.
Locally measured relative proper motions were assembled together on a very precise extended relative reference frame, defined by the mean motion of photometrically selected Galactic Disk stars. External galaxies and the Hipparcos Catalogue were used to transform the proper motions into the ICRS.
Two samples of 3822 LMC stars and 964 SMC stars were selected from the catalog to measure the mean proper motion of the Magellanic Clouds. The results obtained are: (μα cos δ, μδ) LMC ± (1.89, +0.39) + (0.27, 0.27) mas yr-1and (μα cos δ, μδ) SMC = (0.98, − 1.01)±(0.30, 0.29) mas yr -1. We have also obtained a much more precise proper motion of the SMC with respect to the LMC: (μα cos δ, μ δ)SMC–LMC = (−0.91, −1.49) ± (0.16, 0.15) mas yr-1, which was used to obtain new independent and more precise proper motions for the SMC, based on more accurate LMC proper motions of other authors, and also to measure the relative velocity of SMC with respect to LMC within 54 km s-1, similar in precision to other ground- and space-based works.
After comparison with previous results and the analysis of the orbit of the Clouds obtained from our proper motions, we conclude that our numbers are consistent with the Clouds being born and formed as separate entities, which later joined in a temporary binary state for the past few Gigayears, being recently disrupted by the Milky Way in their most recent perigalaticon passage about 200 Myr ago. The Clouds orbits are marginally bound to the Milky Way, possibly following a very ellongated but still periodic orbit around the Galaxy.
|Adviser||William van Altena|
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