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Using the HST ACS, we have obtained deep optical images reaching well belowthe oldest main sequence turnoff in fields on the southeast minor-axis of theAndromeda Galaxy, 35 kpc from the nucleus. These data probe the star formationhistory in the extended halo of Andromeda -- that region beyond 30 kpc thatappears both chemically and morphologically distinct from the metal-rich,highly-disturbed inner spheroid. The present data, together with our previousdata for fields at 11 and 21 kpc, do not show a simple trend toward older agesand lower metallicities, as one might expect for populations further removedfrom the obvious disturbances of the inner spheroid. Specifically, the meanages 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 modelof the 35 kpc population, one third of the stars are younger than 10 Gyr, whileonly ~10% of the stars are truly ancient and metal-poor. The extended halo thusexhibits clear evidence of its hierarchical assembly, and the contribution fromany classical halo formed via early monolithic collapse must be small.

The Extended Star Formation History of the Andromeda Spheroid at 35 Kpc on the Minor Axis