We present a color-magnitude diagram ( CMD ) for a minor-axis field in the halo of the Andromeda galaxy ( M31 ) , 51 arcmin ( 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 \sim 3 \times 10 ^ { 5 } 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 \chi ^ { 2 } fitting to a finely-spaced grid of calibrated stellar-population models .
We find that the M31 halo contains a major ( \sim 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 .