2003ApJ...592L..17B

We present a color-magnitude diagram (CMD) for a minor-axis field in the halo of the Andromeda galaxy (M31), 51' (11 kpc) from the nucleus. These observations, taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, are the deepest optical images yet obtained, attaining 50% completeness at m_V=30.7 mag. The CMD, constructed from ∼3x10⁵ stars, reaches more than 1.5 mag fainter than the old main-sequence turnoff. Our analysis is based on direct comparisons to ACS observations of four globular clusters through the same filters, as well as χ² fitting to a finely spaced grid of calibrated stellar-population models. We find that the M31 halo contains a major (∼30% by mass) intermediate-age (6-8 Gyr) metal-rich ([Fe/H]>-0.5) population, as well as a significant globular cluster age (11-13.5 Gyr) metal-poor population. These findings support the idea that galaxy mergers played an important role in the formation of the M31 halo.