AAS 207th Meeting, 8-12 January 2006
Session 135 A New Look at Andromeda
Poster, Wednesday, 9:20am-6:30pm, January 11, 2006, Exhibit Hall

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[135.04] Dynamical Constraints on the Disk and Spheroid of the Andromeda Spiral Galaxy from Keck Spectroscopy

D. B. Reitzel (UCLA), J. S. Kalirai, P. Guhathakurta (UCSC), R. M. Rich (UCLA), K. M. Gilbert (UCSC), S. R. Majewski, J. C. Ostheimer (UVa), M. C. Cooper (UCB), R. J. Patterson (UVa)

We present spectroscopy of M31 red giant branch (RGB) stars acquired with DEIMOS and LRIS on the Keck telescopes. Using a combination of photometric/spectroscopic diagnostics, we isolate bona fide M31 RGB stars from the foreground Galactic dwarf star population. We use kinematics to directly measure the spheroid/disk ratio, constrain models of the surface brightness profiles of M31's bulge, disk, and halo, and directly test the claim of a dominant disk in M31. In addition, we place limits on overall spheroid rotation along the major and minor axes of M31. Two major-axis fields, located at projected distances of 34 kpc SW and 15 kpc NE of M31's center, show clear evidence for a cold disk-like population, yet also demonstrate that a significant fraction of the stars in each of these fields belong to a hot, non-rotating spheroid-like population. The metallicity of the stars in each field is derived by interpolating their location in the (I, V-I) color-magnitude diagram with a finely spaced grid of theoretical isochrones. The metallicity distribution function of the disk and spheroid populations both contain metal-rich stars. Two SE minor-axis fields located at projected distances of 12 and 30 kpc show no evidence for a large stellar component in a cold disk, placing upper limits on the contribution of M31's disk at large radii.

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 University of California and NASA. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

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