Using the HST ACS , we have obtained deep optical images reaching well below the oldest main sequence turnoff in fields on the southeast minor-axis of the Andromeda Galaxy , 35 kpc from the nucleus .
These data probe the star formation history in the extended halo of Andromeda – that region beyond 30 kpc that appears both chemically and morphologically distinct from the metal-rich , highly-disturbed inner spheroid .
The present data , together with our previous data for fields at 11 and 21 kpc , do not show a simple trend toward older ages and lower metallicities , as one might expect for populations further removed from the obvious disturbances of the inner spheroid .
Specifically , the mean ages and [ Fe/H ] values at 11 kpc , 21 kpc , and 35 kpc are 9.7 Gyr and -0.65 , 11.0 Gyr and -0.87 , and 10.5 Gyr and -0.98 , respectively .
In the best-fit model of the 35 kpc population , one third of the stars are younger than 10 Gyr , while only \sim 10 % of the stars are truly ancient and metal-poor .
The extended halo thus exhibits clear evidence of its hierarchical assembly , and the contribution from any classical halo formed via early monolithic collapse must be small .