AAS 205th Meeting, 9-13 January 2005
Session 141 Our Friendly Neighbors: M31 and M33
Poster, Thursday, January 13, 2005, 9:20am-4:00pm, Exhibit Hall

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[141.13] A Spectroscopic Survey of M31 Red Giants with Keck/DEIMOS: The Andromeda Halo at Intermediate Radii

S.R. Majewski (UVa), J. Kalirai, K. Gilbert, P. Guhathakurta (UCSC), J. Clem (UVic), M. Cooper (UCB), J. Hesser (HIA/NRC), C. Luine (UCSC), D. Reitzel, R. M. Rich (UCLA), P. Stetson (HIA/NRC)

We present new results from continuing imaging and spectroscopic observations of red giants in the halo of the Andromeda spiral galaxy (M31). The synergy of deep wide-field imaging with CFHT/MegaCam and Subaru/SuprimeCam, ultra-deep imaging with HST/ACS, and multiobject spectroscopy with Keck/DEIMOS provides a unique perspective into the formation history and stellar population of M31. Specifically, the Keck/DEIMOS spectrosopic data allow us to isolate red giants in the halo of M31 and eliminate foreground Galactic contamination (mostly M dwarfs - see poster by K. Gilbert et al.) through radial velocity measurements and a variety of spectral and photometric diagnostics. The spectroscopy also yields the chemical composition of the giants, which combined with their kinematical and spatial information provides a means to study coherent groups of stars in the halo and halo-outer disk transition region of M31. We present densely sampled maps of faint red giants in this region, characterize how smooth the halo is, and also identify possible substructure (see also poster by P. Guhathakurta). The fields presented in this poster were chosen to overlap with recent ultra-deep HST/ACS observations of the main-sequence turnoff of M31. We attempt to link the velocities and metallicities of the red giant population with age information from the main-sequence turnoff to examine the age spread in the intermediate-radius halo of M31.

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. This work is supported both by NSF grant AST-0307966 and HST grant GO-10265.02.

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