The Gas Dynamics of Shell Galaxies
  It is widely accepted that shell galaxies form as a result of a merger between an elliptical and a small disk galaxy. Simulations of the stellar component have shown that the shells are created either by ``phase-wrapping'' of debris on nearly radial orbits (Quinn 1984), or by ``spatial-wrapping'' of matter in thin disks (Dupraz & Combes 1987; Hernquist & Quinn 1989; Quillen etal 1993). However, recent HI observations of several shell galaxies (Schiminovich etal 1994, 1995) revealed the presence of gaseous shells displaced from the stellar ones, which questioned the validity of the ``phase-wrapping'' mechanism. Our numerical simulations indicate that these new observations can actually be accommodated within the standard picture for the formation of shell galaxies. 
F. Combes and V. Charmandaris