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 > MK > −18 mag (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 ultracompact 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 Echelle Spectrograph and Imager 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^(−1). 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 dE 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. UCD objects appear to be more similar to massive compact star clusters than to small galaxies. Our dynamical study of low-mass dE galaxies provides further constraints on the processes that shape some of the smallest and most-numerous galaxies in the Universe.