An investigation of the expected frequency of ring galaxies is conducted in the light of the theory of their formation by the collision process involving a disk and a spherical galaxy, using the impulsive approximation. It is found that very favourable collision parametrs (in terms of the impact parameter, orientation, and energy changes) are required for the formation of ring galaxies.
Results ... [Show full abstract] indicate that if the expected frequency of fairly well-defined ring galaxies is measured with respect to regions of high density (in terms of population of galaxies) and using the average distance between galaxies corresponding to these regions for its determination, then it comes out to be of the order of 0.01% of spirals. An even higher frequency is obtained if we take the messy and ill-defined rings into account. The results obtained are quite compatible with the observational values.
But, however, if we consider a region of normal density (where collisions are rare) for frequency determinations, the expected frequency goes down by few orders of magnitude. Thus stray-hyperbolic encounters are too scarce to explain the formation of ring galaxies. This indicates that most of these interacting pairs must have already been bound doubles, whose orbits are such as to have brought about only now the interpenetrating encounters leading to ring formation.