Metallicity and Kinematics of M31’s Outer Stellar Halo from a Keck Spectroscopic Survey

We present multislit CaII near-infrared triplet spectroscopy with the Keck10-m telescope/LRIS of 99 M31 halo red giant candidates in the apparent magrange 200.6 dex,spans the full 2dex range over which [Fe/H] measurement methods are calibrated,and has a mean/median value around <[Fe/H]>=-1.9 to -1.1dex and possibly highersince the high-[Fe/H] end of the distribution is not well constrained by oursample. Our data on M31 field halo giants suggest that they are more metal-richthan their Milky Way counterparts, similar to M31 and Milky Way globularclusters, consistent with a scenario in which the halo is built from accretionof small stellar subsystems. There are 4 stars in the secure M31 sample withunusually strong Ca lines (>~solar [Fe/H]) at a common velocity of -340km/sclose to M31's systemic velocity, similar to what might be expected for M31disk giants on the minor axis. The M31 disk would have to be large and/orwarped to account for these stars. More likely, these stars are metal-richdebris from a past accretion event in M31's halo.Comment: 98 pages, including 4 tables and 24 figures; Accepted for publication in the July, 2002 edition of the Astronomical Journa