2006ApJ...648..389K

We present spectroscopic observations of red giant branch (RGB) stars over a large expanse in the halo of the Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck II 10 m telescope. Using a combination of five photometric/spectroscopic diagnostics–(1) radial velocity, (2) intermediate-width DDO51 photometry, (3) Na I equivalent width (surface gravity-sensitive), (4) position in the color-magnitude diagram, and (5) comparison between photometric and spectroscopic [Fe/H] estimates–we isolate over 250 bona fide M31 bulge and halo RGB stars located in 12 fields ranging from R=12 to 165 kpc from the center of M31 (47 of these stars are halo members with R>60 kpc). We derive the M31 spheroid (bulge and halo) metallicity distribution function and find it to be systematically more metal-poor with increasing radius, shifting from <[Fe/H]≥-0.47±0.03 (σ=0.39) at R<20 kpc to <[Fe/H]≥-0.94±0.06 (σ=0.60) at R∼30 kpc to <[Fe/H]≥-1.26±0.10 (σ=0.72) at R>60 kpc, assuming [α/Fe]=0.0. These results indicate the presence of a metal-poor RGB population at large radial distances out to at least R=160 kpc, thereby supporting our recent discovery of a stellar halo in M31 (structural component with an R^–2 power-law surface brightness profile). This component has a distinct metallicity distribution from M31's bulge. If we assume an α-enhancement of [α/Fe]=+0.3 for M31's halo, we derive <[Fe/H]≥-1.5±0.1 (σ=0.7). Therefore, the mean metallicity and metallicity spread of this newly found remote M31 RGB population are similar to those of the Milky Way halo.