3. The Merger Rate
While it is now well established that interacting and merging galaxies exist, we do not know whether they comprise only an interesting sidelight or the fundamental mechanism for determining the structure and dynamics of most galaxies. In order to make this estimate for the particular types of interactions that can form a polar ring we need to know: 1) the fraction of S0 galaxies that currently appear to have polar rings, 2) the correction for non-optimal viewing orientations, and 3) the correction for possible dimming or limited lifetime of polar-rings. Whitmore et al. (1990; hereafter PRC for Polar Ring Catalog) find that about 5% of all S0 galaxies have, or have had in the past, a polar ring bright enough to make it into the catalog. Since it is likely that accretion events with the correct impact parameters to form a polar-ring galaxy are relatively rare, the number of accretion events for S0 galaxies in general is certainly much larger than this 5%.