The Formation of Compact Elliptical Galaxies in The Vicinity of A
  Massive Galaxy: The Role of Ram-pressure Confinement
  Compact ellipticals (cEs) are outliers from the scaling relations of
early-type galaxies, particularly the mass-metallicity relation which is an
important outcome of feedback. The formation of such low-mass, but metal-rich
and compact, objects is a long-standing puzzle. Using a pair of high-resolution
N-body+gas simulations, we investigate the evolution of a gas-rich low-mass
galaxy on a highly radial orbit around a massive host galaxy. As the infalling
low-mass galaxy passes through the host's corona at supersonic speeds, its
diffuse gas outskirts are stripped by ram pressure, as expected. However, the
compactness increases rapidly because of bursty star formation in the gas
tidally driven to the centre. The metal-rich gas produced by supernovae and
stellar winds is confined by the ram pressure from the surrounding environment,
leading to subsequent generations of stars being more metal-rich. After the gas
is depleted, tidal interactions enhance the metallicity further via the
stripping of weakly bound, old, and metal-poor stars, while the size of the
satellite is changed only modestly. The outcome is a metal-rich cE that is
consistent with observations. These results argue that classical cEs are
neither the stripped remnants of much more massive galaxies nor the merger
remnants of normal dwarfs. We present observable predictions that can be used
to test our model.
