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Determining Photometric Metallicities of dSph Stellar Populations

If there are so few upper red-giant branch stars in the SDSS-discovered dwarf galaxies, how can we find the true population structure without extensive spectroscopy? We review recent photometric and spectroscopic studies of the Ultra-Faint Dwarf Galaxies, and determine a new method of estimating [Fe/H] with a combination of Washington and Stromgren filters, using Bootes I dSph as an example. We can use the CT1by filters alone to achieve 0.3 dex resolution in [Fe/H], and 0.5 Gyr resolution in age. Both the Washington and Stromgren filters, C and v, are sensitive to CN-variations; however, in stars with a large deficiency of heavy elements the CN bands are weak and not important. The [Fe/H]-sensitivity of the Washington and Stromgren combination is at least twice as great as the SDSS filters, and this work maintains that resolution on the lower red-giant branch, where other calibrations fail.