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

We present first results from a spectroscopic survey designed to examine the metallicity and kinematics of individual red giant branch stars in the outer halo of the Andromeda spiral galaxy (M31). This study is based on multislit spectroscopy with the Keck II 10-m telescope and Low Resolution Imaging Spectrograph of the Ca ii near-infrared triplet in 99 M31 halo candidates in a field at R = 19 kpc on the SE minor axis with brightnesses from 20 < I < 22. The spectra are used to isolate M31 halo red giants from foreground Milky Way dwarf stars, faint compact background galaxies, and M31 disk giants. The observed distribution of radial velocities is well fit by an equal mix of foreground Milky Way dwarf stars, drawn from a standard Galactic model and with velocities v . 0 km s, and M31 halo giants represented by a Gaussian of width σ v ∼ 150 km s centered on its systemic velocity of v sys ≈ −300 km s. A secure sample of 29 M31 red giant stars is identified on the basis of radial velocity (v < −220 km s), and, in the case of four intermediate velocity stars (−160 < v < −220 km s), broadband B− I color. For this sample of objects, there is rough agreement between the metallicities derived in independent ways: two different calibrations of the Ca ii absorption line strength and a photometric estimate based on fitting model stellar isochrones to an object’s location in a (B − I, I) color-magnitude 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. This research was carried out as part of DBR’s Ph.D. thesis in the Department of Astronomy & Astrophysics at the University of California at Santa Cruz.