The metal-poor halo of the andromeda spiral galaxy (M31)

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 R12 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 (sigma = 0.39) at R < 20 kpc to <[Fe/H]> = -0.94 +/- 0.06 (sigma = 0.60) at R similar to 30 kpc to <[Fe/H]> = -1.26 +/- 0.10 (sigma = 0.72) at R > 60 kpc, assuming [alpha/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 alpha-enhancement of [alpha/Fe] = +0.3 for M31's halo, we derive <[Fe/H]> = 1.5 +/- 0.1 (sigma = 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.