Global structure and formation of polar-ring galaxies

We present an analysis of structural features of all known galaxies with optical polar rings. We find a clear dichotomy for objects of this peculiar class. Bulge-dominated S0 galaxies possess only short narrow rings, while disk-dominated objects always have wide extended polar rings. We try by gas dynamical simulations to explain such a segregation by dependence of the ring-forming process on different galaxy potentials. It is found that the total mass captured into the ring during an encounter of a host-ring system with a gas-rich spiral galaxy of comparable mass exceeds 109M (or about 10% of all gas in the donor galaxy), which is of the order of that found by observation. The process of gas to gather into a steady-state ring takes approximately (7−9)×108 years. This time is somewhat shorter for rings forming around bulge-dominated galaxies. We also present observational arguments for S0 galaxies with extended rings to be similar to late-type spirals by their photometric properties, while numerical modelling of the extended ring formation suggests that these galaxies must possess massive dark halos as well. In this case, the sizes of the modelled rings turn out large enough (up to 30 kpc in diameter), and the time scale for ring formation is prolonged up to several Gyrs.