Building gas rings and rejuvenating S0 galaxies through minor mergers
  We investigate the effects of minor mergers between an S0 galaxy and a
gas-rich satellite galaxy, by means of N-body/smoothed particle hydrodynamics
(SPH) simulations. The satellite galaxy is initially on a nearly parabolic
orbit and undergoes several periapsis passages before being completely
stripped. In most simulations, a portion of the stripped gas forms a warm dense
gas ring in the S0 galaxy, with a radius of ~6-13 kpc and a mass of ~10^7 solar
masses (Msun). The ring is generally short-lived (<~3 Gyr) if it forms from
prograde encounters, while it can live for more than 6 Gyr if it is born from
counter-rotating or non-coplanar interactions. The gas ring keeps memory of the
initial orbit of the satellite galaxy: it is corotating (counter-rotating) with
the stars of the disc of the S0 galaxy, if it originates from prograde
(retrograde) satellite orbits. Furthermore, the ring is coplanar with the disc
of the S0 galaxy only if the satellite's orbit was coplanar, while it lies on a
plane that is inclined with respect to the disc of the S0 galaxy by the same
inclination angle as the orbital plane of the satellite galaxy. The fact that
we form polar rings as long-lived and as massive as co-planar rings suggests
that rings can form in S0 galaxies even without strong bar resonances. Star
formation up to 0.01 Msun yr^-1 occurs for >6 Gyr in the central parts of the
S0 galaxy as a consequence of the interaction. We discuss the implications of
our simulations for the rejuvenation of S0 galaxies in the local Universe.
