On the Structural Differences between Disk and Dwarf Galaxies

Gas-rich dwarf and disk galaxies overlap in numerous physical quantities thatmake their classification subjective. We report the discovery of a separationbetween dwarfs and disks into two unique sequences in the mass (luminosity)versus scale length plane. This provides an objective classification scheme forlate-type galaxies that only requires optical or near-IR surface photometry ofa galaxy. Since the baryonic Tully-Fisher relation for these samples produces acontinuous relation between baryonic mass and rotational velocity, we concludethat the difference between dwarfs and disks must be because of theirdistribution of stellar light such that dwarfs are more diffuse than diskgalaxies. This structural separation may be due to a primordial differencebetween low and high mass galaxies or produced by hierarchical mergers wheredisks are built up from dwarfs. Structural differences between dwarf and diskgalaxies may also be driven by the underlying kinematics where the strongrotation in disks produces an axial symmetric object that undergoes highlyefficient star formation in contrast to the lower rotation, more disorderedmotion of dwarfs that produces a diffuse, triaxial object with a history ofinefficient star formation.Comment: 16 pages, 2 figures, AJ in press, AASTeX5.