NOAO >   Observing Info >   Approved Programs >   2006A-0578

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Proposal Information for 2006A-0578


PI: Mei-Yin Chou, University of Virginia, mc6ss@virginia.edu
Address: Astronomy Department, P.O. Box 3818, Charlottesville, VA 22936-0818, USA

CoI: Steven Majewski, University of Virginia
CoI: Verne Smith, NOAO
CoI: Katia Cunha, NOAO
CoI: David L. Nidever, University of Virginia
CoI: Kathryn Johnston, Wesleyan University
CoI: Sabrina Pakzad, University of Virginia
CoI: Richard J. Patterson, University of Virginia
CoI: Michael F. Skrutskie, University of Virginia
CoI: David Law, Caltech

Title: Testing The Sagittarius-LMC Collision Hypothesis

Abstract: \vskip -0.2in One of the most perplexing, unsolved mysteries regarding the tidally disrupting Sagittarius (Sgr) dSph galaxy is how it has remained in its present, very destructive orbit for a Hubble time. A second mystery is why, given such a destructive orbit, Sgr only has tidal tails corresponding to ~ 2 Gyr of stripping (as mapped, e.g., by its M giants). Our hypothesis is that ~ 2 Gyr ago Sgr was bumped from a large, safe orbit into its present orbit by collision with the Large Magellanic Cloud (LMC). Backwards 3-body integration of the LMC and Sgr orbits shows a close interaction 2 Gyr ago, and one sufficient to create the orbit change. Moreover, we have discovered another M giant tidal stream in the outer (>50 kpc) halo wrapping 360^\circ around the Milky Way. This stream has positions and velocities consistent with debris from a (now ``missing") satellite orbiting near the Sgr orbital plane in a slightly elliptical orbit that intersects the ``inner" Sgr orbit ~2 Gyr ago. Since M giants in dSphs are rare, the logical conclusion is that this outer M giant stream corresponds to a former Sgr orbit. We propose to test this hypothesis by checking whether M giants in the outer halo debris stream have the peculiar s-process abundance ratios that is the hallmark of Sgr dSph stars and whether, in fact, no other older Sgr debris exists among other inner halo M giants.


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