Resonant stripping as the origin of dwarf spheroidal galaxies
  Dwarf spheroidal galaxies are the most dark matter dominated systems in the
nearby Universe and their origin is one of the outstanding puzzles of how
galaxies form. Dwarf spheroidals are poor in gas and stars, making them
unusually faint, and those known as ultra-faint dwarfs have by far the lowest
measured stellar content of any galaxy. Previous theories require that dwarf
spheroidals orbit near giant galaxies like the Milky Way, but some dwarfs have
been observed in the outskirts of the Local Group. Here we report simulations
of encounters between dwarf disk galaxies and somewhat larger objects. We find
that the encounters excite a process, which we term ``resonant stripping'',
that can transform them into dwarf spheroidals. This effect is distinct from
other mechanisms proposed to form dwarf spheroidals, including mergers,
galaxy-galaxy harassment, or tidal and ram pressure stripping, because it is
driven by gravitational resonances. It may account for the observed properties
of dwarf spheroidals in the Local Group, including their morphologies and
kinematics. Resonant stripping predicts that dwarf spheroidals should form
through encounters, leaving detectable long stellar streams and tails.
