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We present the results of a deep study of the isolated dwarf galaxiesAndromeda XXVIII and Andromeda XXIX with Gemini/GMOS and Keck/DEIMOS. Bothgalaxies are shown to host old, metal-poor stellar populations with nodetectable recent star formation, conclusively identifying both of them asdwarf spheroidal galaxies (dSphs). And XXVIII exhibits a complex horizontalbranch morphology, which is suggestive of metallicity enrichment and thus anextended period of star formation in the past. Decomposing the horizontalbranch into blue (metal poor, assumed to be older) and red (relatively moremetal rich, assumed to be younger) populations shows that the metal rich arealso more spatially concentrated in the center of the galaxy. We usespectroscopic measurements of the Calcium triplet, combined with the improvedprecision of the Gemini photometry, to measure the metallicity of the galaxies,confirming the metallicity spread and showing that they both lie on theluminosity-metallicity relation for dwarf satellites. Taken together, thegalaxies exhibit largely typical properties for dSphs despite their significantdistances from M31. These dwarfs thus place particularly significantconstraints on models of dSph formation involving environmental processes suchas tidal or ram pressure stripping. Such models must be able to completelytransform the two galaxies into dSphs in no more than two pericentric passagesaround M31, while maintaining a significant stellar populations gradient.Reproducing these features is a prime requirement for models of dSph formationto demonstrate not just the plausibility of environmental transformation butthe capability of accurately recreating real dSphs.

A Deep Study of the Dwarf Satellites Andromeda XXVIII & Andromeda XXIX