The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy1,2

Using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, we have obtained deep optical images reaching stars well below the oldest main sequence turnoff in three fields of the Andromeda Galaxy: the spheroid, tidal stream, and outer disk. These observations allow a complete reconstruction of the star formation history in these fields. We analyze the color-magnitude diagrams of the stellar populations in these fields by comparison to a grid of theoretical isochrones calibrated to Galactic globular clusters observed in the same ACS bands. All three fields exhibit an extended star formation history, with a significant fraction of stars younger than 10 Gyr but few stars younger than 4 Gyr. Considered together, the star count maps, radial velocities, and population characteristics of the spheroid argue against some explanations for its intermediate-age, metal-rich population, such as a significant contribution from stars residing in the disk or a chance intersection with the orbit of the tidal stream. Instead, it is likely that these intermediate-age, metal-rich stars are intrinsic to the inner spheroid, whose highly-disturbed structure is clearly distinct from the pressure-supported metalpoor halo that dominates at large distances from the galaxy’s center. The stream and spheroid populations are similar, but not identical, with the mean age in the stream being ∼1 Gyr younger; the similarity between the stream and spheroid populations suggests that the inner spheroid is largely polluted by material stripped from the stream’s progenitor or by objects like this progenitor. The disk population is considerably younger and more metal-rich than the stream and spheroid populations, but not as young as the thin disk population of the solar neighborhood; instead, the outer disk of Andromeda is dominated by intermediate-age stars of age 4–8 Gyr, resembling the Milky Way’s thick disk population. The disk data are inconsistent with a population dominated by ages older than 10 Gyr, and in fact do not require any stars older than 10 Gyr. Subject headings: galaxies: evolution – galaxies: stellar content – galaxies: halos – galaxies: spiral – galaxies: individual (M31)