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a) Morphological Type of the Central Component

The central component of all the polar-ring systems in Categories A and B appear to be diffuse systems; either S0 disks or ellipticals. In no case is the central component a spiral galaxy. This can probably be explained by the need for one, or both, of the two components to be free of gas since the orbits intersect. In principle, both components could be gas free. The fact that the outer component contains gas in all seven of the well-observed systems indicates that gaseous dissipation is probably required to form the ring. If the outer ring must necessarily be gaseous in order to settle to a disk, then the inner component cannot be gaseous because of the intersecting orbits.

In six of the seven well-studied polar-ring galaxies the central component has turned out to be a rapidly rotating S0 disk. The exception is AM 2020-504 which has a slowly rotating elliptical for a central component (Whitmore et al. 1987b; note, however, that even this galaxy has a rapidly rotating system in the inner few arcseconds of the central elliptical). The component aligned with the minor axis of dust-lane ellipticals rarely shows an extended luminous component. Perhaps this difference is just due to a difference in the gravitational potential of the two types of systems. For example, the presumably more flattened potential of the S0 systems may be able to stabilize the ring at greater radii than in an elliptical galaxy. The presence of the ring in AM 2020-504 may indicate that this elliptical galaxy is flatter than a typical elliptical.

Another interesting point is that none of the central components appear to have a bar-like structure (Galletta 1990). Perhaps the degree of asymmetry the bar would produce in the gravitational potential is enough to disrupt the formation of a ring in these cases. Computer modelling of this situation should be able to answer this question.