The Star Formation History in Andromeda's Diffuse Stellar HaloT.M. Brown, E. Smith, H.C. Ferguson (STScI), P. Guhathakurta (UCSC), R.M. Rich (UCLA), J. Kalirai (UCSC), A. Renzini (INAF), & A.V. Sweigart (NASA/GSFC)
We present a color-magnitude diagram from deep Hubble Space Telescope observations of a minor-axis field 21 kpc away from the center of Andromeda. In both star counts and chemical abundances, this field represents a transition point between the highly-disturbed inner spheroid that dominates within 15 kpc and the diffuse population that dominates beyond 30 kpc. Our observations reach well below the oldest main sequence turnoff in the population, allowing a complete reconstruction of the star formation history in this field. We compare the current observations to our previous observations in the inner spheroid, outer disk, and tidal stream, and consider implications for the formation history of these structural components.