AAS 201st Meeting, January, 2003
Session 14. Galaxy - Contents
Poster, Monday, January 6, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[14.10] Red Giants in the Outer Halo of M31: Keck Spectroscopy in the Vicinity of G1

D.B. Reitzel (University of California at Los Angeles), P. Guhathakurta (UCO/Lick Observatory), R.M. Rich (University of California at Los Angeles)

We present results from an ongoing spectroscopic survey of individual red giant branch (RGB) stars in the halo of the Andromeda spiral galaxy (M31). These results are based on multi-slit spectroscopy with the Keck II telescope and Low Resolution Imaging Spectrometer of the Ca II near infrared triplet in about 50 M31 halo candidates with brightnesses in the range 20 < I < 22 in a field around the luminous globular cluster G1. G1 is located at a projected distance of 34 kpc from the nucleus near the major axis of M31. Metallicity measurements are made on a star-by-star basis using two independent methods: the strength of the Ca II absorption lines and the location in the V-I vs I color-magnitude diagram. The spectra are used to isolate M31 halo RGB members from foreground Milky Way dwarf stars on the basis of radial velocity, and this yields a clean sample of M31 halo stars. This clean sample has a large spread in metallicity, -2.8 < Fe/H < 0.0. The metal poor population, [Fe/H] < -1, displays a wide range of velocities, -500 km/s < v < 50 km/s, while the metal-rich population, [Fe/H] ~-0.7 dex is concentrated near v ~-480 km/s. The velocity of these metal-rich stars is consistent with 21-cm neutral hydrogen line observations of M31's disk 30 kpc from the nucleus parallel to the major axis near the location of G1 (Cram et al. 1980). No concentration of stars is detected at the velocity of the globular cluster G1; we might have expected to detect tidal debris if G1 were the core of a dwarf galaxy being accreted by 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. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Partial support of this work provided by NASA grant GO-9099 through the Space Telescope Science Institute.

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