Metallicity and Kinematics of M31's Outer Stellar Halo from a Keck Spectrocscopic Survey

We present spectroscopy of 99 M31 halo red giant candidates in the apparent mag range 20 0.6 dex, spans the full 2 dex range over which [Fe/H] measurement methods are calibrated, and has a mean/median value around = -1.9 to -1.1 dex and possibly higher since the high-[Fe/H] end of the distribution is not well constrained by our sample. Our data on M31 field halo giants suggest that they are somewhat more metal-rich than their Milky Way counterparts, similar to M31 and Milky Way globular clusters, broadly consistent with a scenario in which the halo is built from the accretion of small stellar subsystems. There are 4 stars in the secure M31 sample with unusually strong Ca lines (>~ solar [Fe/H]) to a common velocity of -340 km/s close to M31's systemic velocity. These 4 stars may be metal-rich debris from a past accretion event in the M31 halo