Abstract
An explanation for the low frequency of ring galaxies is investigated theoretically on the basis of the collisional theory, according to which the ring galaxy is the aftermath of a collision between a disk and another galaxy. Impulsive approximation calculations indicate that if the expected frequency of ring galaxies is measured with respect to densely populated regions, using the value of the collisional frequency corresponding to such regions, then it comes out to be of the order of 0.01% of spirals, compatible with the observed frequency of ring galaxies. But,however,if we consider sparse regions, populated by field galaxies. for frequency determinations, the expected frequency goes down by few orders of magnitude. Thus stray, hyperbolic encounters are too scarse 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
