NOAO >   Observing Info >   Approved Programs >   2012A-0305

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Proposal Information for 2012A-0305


PI: Allyson A. Sheffield, Columbia University, asheffield@astro.columbia.edu
Address: Astronomy Department, 550 W. 120th St., New York, NY 10027, US

CoI: Kathryn V. Johnston, Columbia University
CoI: Steven R. Majewski, University of Virginia
CoI: Katia Cunha, National Optical Astronomy Observatory
CoI: Verne V. Smith, National Optical Astronomy Observatory

Title: Exploring the Origin of M Giant Stars in the Nearby Galactic Halo

Abstract: A recurring question in Milky Way formation scenarios is the origin of stars in the Galactic halo. Several formation mechanisms have been proposed: (1) stars form \it in situ in the halo during the dissipational collapse of the protogalaxy; (2) stars form in the disk and are \it kicked-out by subsequent merger events; (3) stars form in separate dark matter halos that are later \it accreted by the Milky Way. To date, the spatial and velocity distributions of the stellar halo suggest that it largely formed by accretion in the outer parts, and hint at a transition to dominance by an \it in situ or \it kicked out population further in, in agreement with recent hydrodynamic models of galaxy formation. However, in a recent study of 34 nearby (<10 kpc) M giant stars with velocities high enough to imply membership of the stellar halo we found almost half the sample (16 stars) had abundance patterns that closely resembled those in Milky Way satellites (and hence an \it accreted population), and were distinct from the dominant stellar halo or disk trends. This result re-opens the debate on the origin of the inner halo, in particular because M giants are relatively metal rich and hence expected to be biased towards the \it kicked out disk population. This proposal requests further observing time to gather a more significant sample of abundances with which to address this apparent contradiction.


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