PI: Abhijit Saha, NOAO, email@example.com
Address: NTSS, PO Box 26732 (950 N. Cherry Ave), Tucson, AZ 85726-6732, USA
CoI: Edward Olszewski, Steward Observatory, University of Arizona
CoI: Andrew Dolphin, Steward Observatory, University of Arizona
Title: Breaking the age-metallicity degeneracy in the ``deepest ever'' HST M31 halo field
Abstract: The formation mechanism of galaxy halos has been the source of considerable debate. There is evidence pointing toward both gravitational collapse and accretion as sources of some stars, but the relative importance of these sources are unknown. This question can be addressed with measurements of metallicities and ages of halo stars. Recent ultra-deep HST/ACS observations of a field in the M31 halo provide the necessary photometry for measuring these quantities. However, the published analysis of this important data set is of only limited value due to the treatment of metallicity effects and the age- metallicity degeneracy. We propose to obtain metallicities of red giants in this field using Washington photometry, thus providing an external constraint on the star formation history.
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