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J. S. Kalirai, K. M. Gilbert, P. Guhathakurta (UCSC), S. R. Majewski, J. C. Ostheimer (UVa), D. B. Reitzel, R. M. Rich (UCLA), M. C. Cooper (UCB), R. J. Patterson (UVa)
Simulations of hierarchical galaxy formation suggest that large galaxies such as the Milky Way and M31 should contain extended stellar halos that are chemically distinct (more metal poor) from the inner bulge. Yet, no such population has yet been discovered in our nearest neighbor, M31. We report on the first results from a large spectroscopic survey of red giant branch (RGB) stars in the outskirts of the Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck II 10-m telescope. Using a combination of photometric and spectroscopic diagnostics, we isolate bonafide M31 halo RGB stars located in fields ranging from R = 12 - 160 kpc from the center of M31. Out to 30 kpc, we confirm earlier studies and find that M31 is dominated by a metal-rich, R1/4 surface brightness (de Vaucouleur) profile. However, beyond this distance, the brightness profile of M31 RGB stars lies well above an outward extrapolation of the inner bulge-like profile and is consistent with an R-2 projected profile (i.e., a smooth halo). We measure both spectroscopic and photometric metallicities for this population and find that, in fact, the outer halo of M31 is metal-poor relative to the inner bulge.
Data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. We are grateful to the DEIMOS instrument team at UCSC. JSK is supported by NASA through Hubble Fellowship grant HF-01185.01-A. This work is also supported both by NSF grant AST-0307966 and HST grant GO-10265.02.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.