0 Keck studies of M 31 ’ s stellar halo ∗

We present Keck 10-meter / LRIS spectra of candidate red giants in the halo of M31, located at a projected radius of R = 19 kpc on the minor axis. These spectroscopic targets have been selected using a combination of UBRI-based and morphological screening to eliminate background galaxies. Radial velocity measurements are used to separate M31 halo giants from foreground Milky Way dwarf stars, M31 disk stars, and residual background galaxies. The metallicity of each M31 halo giant is measured using standard photometric and spectroscopic techniques, the latter based on the strength of the Ca II triplet. The various [Fe/H] estimates are in rough agreement with one another. The data reveal a large spread (> 2 dex) in [Fe/H] in M31’s halo; there is no strong radial [Fe/H] gradient. LRIS and HIRES spectra are also presented for red giants in five dwarf spheroidal satellites of M31: And I, And III, AndV, AndVI, and AndVII. There appears to be a significant metallicity spread in AndVI and possibly in And I. The new radial velocity data on these outer dwarfs are used to constrain the total mass of M31: the best estimate is under 10 M⊙, somewhat less than the best estimate for the Milky Way.