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

Abstract

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, at 11, 21, and 35 kpc, the mean ages are 9.7, 11.0, and 10.5 Gyr, respectively, and the mean [Fe/H] values are 0.65, 0.87, and 0.98, respectively. In the best-fit model of the 35 kpc population, one-third of the stars are younger than 10 Gyr, whereas only ∼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.