Long-Time Evolution of Gas-Free Disc Galaxies in Binary Systems
  We present the results of several detailed numerical N-body simulations of
the dynamical interactions of two equal mass disc galaxies. Both galaxies are
embedded in spherical halos of dark matter and contain central bulges. Our
analysis of the dynamical evolution of the binary system focuses on the
morphological evolution of the stellar distribution of the discs. The satellite
galaxy has coplanar or polar disc orientation in relation to the disc of the
primary galaxy and their initial orbits are prograde eccentric ($e=0.1$,
$e=0.4$ or $e=0.7$). Both galaxies have mass and size comparable to the Milky
Way. We show that the merger of the two disc galaxies, depending on the
relative orientation of the discs, can yield either a disc or lenticular
remnant, instead of an elliptical one. These are the first simulations in the
literature to show the formation of S0-like galaxies from protracted binary
galaxy interactions. Additionally, we demonstrate that the time to merger
increases linearly with the initial apocentric distance between the galaxies,
and decreases with the initial orbital eccentricity. We also show that the
tidal forces of the discs excite transient $m=1$ and $m=2$ wave modes, i.e.,
lopsidedness, spiral arms, and bars. However, after the merging of the discs,
such instabilities fade completely, and the remnant is thicker and bigger than
the original discs. The maximum relative amplitude of these waves is at most
about 15 times greater compared to the control case. of these two wave modes.
Finally, the disc settles down quickly, after the merger, in less than one
outer disc rotation period.
