Type I shell galaxies as a test of gravity models
  Shell galaxies are understood to form through the collision of a dwarf galaxy
with an elliptical galaxy. Shell structures and kinematics have been noted to
be independent tools to measure the gravitational potential of the shell
galaxies. We compare theoretically the formation of shells in Type I shell
galaxies in different gravity theories in this work because this is so far
missing in the literature. We include Newtonian plus dark halo gravity, and two
non-Newtonian gravity models, MOG and MOND, in identical initial systems. We
investigate the effect of dynamical friction, which by slowing down the dwarf
galaxy in the dark halo models limits the range of shell radii to low values.
Under the same initial conditions, shells appear on a shorter timescale and
over a smaller range of distances in the presence of dark matter than in the
corresponding non-Newtonian gravity models. If galaxies are embedded in a dark
matter halo, then the merging time may be too rapid to allow multi-generation
shell formation as required by observed systems because of the large dynamical
friction effect. Starting from the same initial state, in the dark halo model
the observation of small bright shells should be accompanied by large faint
ones, while for the case of MOG, the next shell generation patterns iterate
with a specific time delay. The first shell generation pattern shows a
degeneracy with the age of the shells and in different theories, but the
relative distance of the shells and the shell expansion velocity can break this
degeneracy.
