Ring galaxies from off-centre collisions
  We investigate the formation of RE galaxies (i.e. of collisional ring
galaxies with an empty ring), with N-body/SPH simulations. The simulations
employ a recipe for star formation (SF) and feedback that has been shown to be
crucial to produce realistic galaxies in a cosmological context. We show that
RE galaxies can form via off-centre collisions (i.e. with a non-zero impact
parameter), even for small inclination angles. The ring can be either a
complete ring or an arc, depending on the initial conditions (especially on the
impact parameter). In our simulations, the nucleus of the target galaxy is
displaced from the dynamical centre of the galaxy and is buried within the
ring, as a consequence of the off-centre collision. We find that the nucleus is
not vertically displaced from the plane of the ring. We study the kinematics of
the ring, finding agreement with the predictions by the analytic theory. The SF
history of the simulated galaxies indicates that the interaction enhances the
SF rate. We compare the results of our simulations with the observations of Arp
147, that is the prototype of RE galaxies.
