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

Using the Advanced Camera for Surveys on the Hubble Space Telescope, we haveobtained deep optical images reaching stars well below the oldest main sequenceturnoff in the spheroid, tidal stream, and outer disk of the Andromeda Galaxy.We have reconstructed the star formation history in these fields by comparingtheir color-magnitude diagrams to a grid of isochrones calibrated to Galacticglobular clusters observed in the same bands. Each field exhibits an extendedstar formation history, with many stars younger than 10 Gyr but few youngerthan 4 Gyr. Considered together, the star counts, kinematics, and populationcharacteristics of the spheroid argue against some explanations for itsintermediate-age, metal-rich population, such as a significant contributionfrom stars residing in the disk or a chance intersection with the stream'sorbit. Instead, it is likely that this population is intrinsic to the innerspheroid, whose highly-disturbed structure is clearly distinct from thepressure-supported metal-poor halo that dominates farther from the galaxy'scenter. The stream and spheroid populations are similar, but not identical,with the stream's mean age being ~1 Gyr younger; this similarity suggests thatthe inner spheroid is largely polluted by material stripped from either thestream's progenitor or similar objects. The disk population is considerablyyounger and more metal-rich than the stream and spheroid populations, but notas young as the thin disk population of the solar neighborhood; instead, theouter disk of Andromeda is dominated by stars of age 4 - 8 Gyr, resembling theMilky Way's thick disk. The disk data are inconsistent with a populationdominated by ages older than 10 Gyr, and in fact do not require any stars olderthan 10 Gyr.