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P. Guhathakurta, K. M. Gilbert, J. S. Kalirai (UCO/Lick Obs, UCSC), J. C. Ostheimer, S. R. Majewski, R. J. Patterson (UVa), R. M. Rich, D. B. Reitzel (UCLA), M. C. Cooper (UC Berkeley)
Understanding galaxy formation is a key goal of modern cosmology. Our Galaxy's spheroidal halo of old stars halo serves as a fossil record of its history of formation via accretion of smaller galaxies. Studies of our neighbour, the Andromeda galaxy, have concluded that its spheroid contains chemically enriched (``metal-rich'') stars out to a radius of 30 kpc with an exponential fall-off in density and thus resembles a galactic ``bulge''. Were the true halo of Andromeda to be found, our detailed yet global view of its stellar dynamics, substructure, chemical abundance, and age distribution would constrain hierarchical halo formation models in a direct new way. We report here on the discovery of this hitherto elusive component: a halo of metal-poor Andromeda stars, distinct from its bulge, with a power-law brightness profile extending beyond R=160 kpc. This is 3-5 times larger than any previously mapped Andromeda spheroidal/disk component. The Milky Way and Andromeda halo radii together span more than one-third of the distance between them. This suggests that stars occupy a substantial volume fraction of our Local Group, and possibly all galaxy groups.
P.G. acknowledges support from NSF grant AST-0307966. Data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among Caltech, the Univ of California and NASA. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.