Bridging the gap between low and high mass dwarf galaxies
  While the dark matter content within the most massive giant and smallest
dwarf galaxies has been probed -- spanning a range of over one million in mass
-- an important observational gap remains for galaxies of intermediate mass.
This gap covers K band magnitudes of approximately -16 > M_K > -18 (for which
dwarf galaxies have B--K ~ 2). On the high mass side of the gap are dwarf
elliptical (dE) galaxies, that are dominated by stars in their inner regions.
While the low mass side includes dwarf spheroidal (dSph) galaxies that are dark
matter-dominated and ultra compact dwarf (UCD) objects that are star-dominated.
Evolutionary pathways across the gap have been suggested but remain largely
untested because the `gap' galaxies are faint, making dynamical measurements
very challenging. With long exposures on the Keck telescope using the ESI
instrument we have succeeded in bridging this gap by measuring the dynamical
mass for five dwarf galaxies with M_K ~ -17.5 (M_B ~ --15.5). With the
exception of our brightest dwarf galaxy, they possess relatively flat velocity
dispersion profiles of around 20 km/s. By examining their 2D scaling relations
and 3D fundamental manifold, we found that the sizes and velocity dispersions
of these gap galaxies reveal continuous trends from dE to dSph galaxies. We
conclude that low-luminosity dwarf elliptical galaxies are dominated by stars,
not by dark matter, within their half light radii. This finding can be
understood if internal feedback processes are operating most efficiently in gap
galaxies, gravitationally heating the centrally-located dark matter to larger
radii. Whereas external environmental processes, which can strip away stars,
have a greater influence on dSph galaxies resulting in their higher dark matter
fractions. Abridged.
