KECK SPECTROSCOPY OF RED GIANTS IN M31's STELLAR HALO

I present results from an ongoing Keck spectroscopic survey of red giant stars in the halo of the Andromeda spiral galaxy (M31) and its dwarf spheroidal satellites. The observed shape of M31's stellar halo, roughly 2:1 in projection, is consistent with rotational flattening: its rotation speed and line-of-sight velocity dispersion are measured to be ν_rot ≈ σ_ν ≈ 150 km s^-1. The M31 stellar halo appears to be denser and/or larger than our Galaxy's halo. Yet, the best estimate of the mass of M31's dark halo, using the most extensive set of dynamical tracers available, suggests that the galaxy is likely to be less massive than our own. A small group of metal-rich stars is seen at a common velocity indicative of substructure in M31's halo or a disk that is very extended and/or warped. The unusual properties of the M31 satellite LGS3, classified as a transition type object between dwarf spheroidals and dwarf irregulars, may be related to its being on a highly eccentric orbit around the parent galaxy. The brightest of the M31 dwarf spheroidal satellites, Cas dSph (or And VII), has an internal velocity dispersion of about 9 km s^-1, which is indicative of a dark matter content consistent with the extrapolation of the properties of other Local Group dwarf spheroidal satellites.