4. Insight into How Other Galaxies Formed
Several classes of objects have some characteristics which are similar to polar-ring galaxies (see Section II). For example, many elliptical galaxies have dust-lanes oriented along their minor axis but have no extended luminous component associated with this material. Could the formation mechanism be the same in both cases but the resulting products appear different because of differences in the gravitational potential of S0 and elliptical galaxies? Another example is for spiral and S0 galaxies which have box-shaped, peanut-shaped, and X-shaped bulges. Whitmore and Bell (1988) have suggested that these systems might represent "failed" polar rings, where the impact was at an intermediate angle rather than being nearly perpendicular.
In the past, only four systems had been observed in enough detail to firmly establish their status as S0 galaxies with polar rings (NGC 2685: Schechter and Gunn 1978; A0136-0801: Schweizer, Whitmore and Rubin 1983, WMS; NGC 4650A: Schechter, Ulrich and Boksenberg 1984, WMS; ESO 415-G26: WMS). Four other galaxies were listed as good candidates by Schweizer, Whitmore and Rubin (1983) along with 14 possibly related galaxies. Observations reported in PRC show that two of these good candidates (UGC 7576 and UGC 9796 [= II Zw 073]) are also S0 galaxies with polar rings. A third candidate from this list, AM 2020-504, turns out to have an elliptical galaxy as the central component (see Section IIIa). The other candidates and related objects have not been observed in enough detail to make this determination.
The PRC provides a large enough sample to examine some of the statistical properties of polar-ring galaxies. What fraction of polar rings are narrow rings and what fraction are more extended annuli? Are the warps similar to those seen in normal galaxies? Is the central component always an S0 galaxy? Is the angle between the two components random, or is there a preference for being nearly perpendicular? These are some of the questions we will address in this article.