A Deep Study of the Dwarf Satellites Andromeda XXVIII & Andromeda XXIX
  We present the results of a deep study of the isolated dwarf galaxies
Andromeda XXVIII and Andromeda XXIX with Gemini/GMOS and Keck/DEIMOS. Both
galaxies are shown to host old, metal-poor stellar populations with no
detectable recent star formation, conclusively identifying both of them as
dwarf spheroidal galaxies (dSphs). And XXVIII exhibits a complex horizontal
branch morphology, which is suggestive of metallicity enrichment and thus an
extended period of star formation in the past. Decomposing the horizontal
branch into blue (metal poor, assumed to be older) and red (relatively more
metal rich, assumed to be younger) populations shows that the metal rich are
also more spatially concentrated in the center of the galaxy. We use
spectroscopic measurements of the Calcium triplet, combined with the improved
precision of the Gemini photometry, to measure the metallicity of the galaxies,
confirming the metallicity spread and showing that they both lie on the
luminosity-metallicity relation for dwarf satellites. Taken together, the
galaxies exhibit largely typical properties for dSphs despite their significant
distances from M31. These dwarfs thus place particularly significant
constraints on models of dSph formation involving environmental processes such
as tidal or ram pressure stripping. Such models must be able to completely
transform the two galaxies into dSphs in no more than two pericentric passages
around M31, while maintaining a significant stellar populations gradient.
Reproducing these features is a prime requirement for models of dSph formation
to demonstrate not just the plausibility of environmental transformation but
the capability of accurately recreating real dSphs.
