On the structural differences between disk and dwarf galaxies

Gas-rich dwarf and disk galaxies overlap in numerous physical quantities, which makes their classification subjective. We report the discovery of a separation of dwarfs and disks into two unique sequences in the mass (luminosity) versus scale length plane. This provides an objective classification scheme for late-type galaxies that only requires optical or near-IR surface photometry of a galaxy. Since the baryonic Tully-Fisher relation for these samples produces a continuous relation between baryonic mass and rotational velocity, we conclude that the difference between dwarfs and disks must be due to their distribution of stellar light such that dwarfs are more diffuse than disk galaxies. This structural separation may be due to a primordial difference between low- and high-mass galaxies or produced by hierarchical mergers in which disks are built up from dwarfs. Structural differences between dwarf and disk galaxies may also be driven by the underlying kinematics, where the strong rotation in disks produces an axially symmetric object that undergoes highly efficient star formation in contrast to the more weakly rotating, more disordered motion of dwarfs that produces a diffuse, triaxial object with a history of inefficient star formation.