  Axis-symmetric ring systems can be identified from the new catalog of
collisional ring galaxies in Madore et al. (2009). These are O-type-like
collisional ring galaxies. Head-on collisions by dwarf galaxies moving along
the symmetric axis were performed through N-body simulations to address their
origins. It was found that the simulations with smaller initial relative
velocities between two galaxies, or the cases with heavier dwarf galaxies,
could produce rings with higher density contrasts. There are more than one
generation of rings in one collision and the lifetime of any generation of
rings is about one dynamical time. It was concluded that head-on penetrations
could explain these O-type-like ring galaxies identified from the new catalog
in Madore et al. (2009), and the simulated rings resembling the observational
O-type-like collisional rings are those at the early stage of one of the
ring-generations.
