  We numerically investigate stellar and gas dynamics in star-forming and
dissipative galaxy mergers between two disk galaxies with specific orbital
configurations. We find that violent relaxation combined with gaseous
dissipation in galaxy merging transforms two disk galaxies into one S0 galaxy
with polar-rings: Both the central S0-like host and the polar-ring component in
a polar-ring galaxy are originally disk galaxies. We also find that morphology
of the developed polar-rings reflects both the initial orbit configuration of
galaxy merging and the initial mass ratio of the two merger progenitor disk
galaxies. Based upon these results, we discuss the origin of the fundamental
observational properties of polar-ring galaxies, such as the prevalence of S0
galaxies among polar-ring galaxies, the rarity of polar-ring galaxies among S0
galaxies, the dichotomy between narrow polar-rings and annular ones, shapes of
polar-ring warps, and an appreciably larger amount of interstellar gas in the
polar-ring component.
