The Extended Star Formation History of the Andromeda Spheroid at Twenty One Kiloparsecs on the Minor Axis

Using the HST ACS, we have obtained deep optical images of a southeast minor-axis field in the Andromeda Galaxy, 21 kpc from the nucleus. In both star counts and metallicity, this field represents a transition zone between the metal-rich, highly-disturbed inner spheroid that dominates within 15 kpc and the metal-poor, diffuse population that dominates beyond 30 kpc. The color-magnitude diagram reaches well below the oldest main-sequence turnoff in the population, allowing a reconstruction of the star formation history in this field. Compared to the spheroid population at 11 kpc, the population at 21 kpc is ~1.3 Gyr older and ~0.2 dex more metal-poor, on average. However, like the population at 11 kpc, the population at 21 kpc exhibits an extended star formation history; one third of the stars are younger than 10 Gyr, although only a few percent are younger than 8 Gyr. The relatively wide range of metallicity and age is inconsistent with a single, rapid star-formation episode, and instead suggests that the spheroid even at 21 kpc is dominated by the debris of earlier merging events likely occurring more than 8 Gyr ago.