Formation of a polar-ring galaxy in a galaxy merger
  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. 
Kenji Bekki