Although studies applying rigidly rotating bar potentials have shed light on the formation of rings in the disks of galaxies, self-consisted simulations provide valuable complementary information:
1) Our results support the standard hypothesis that links rings to resonances, which in most cases are induced by the bar component. More specifically, the outer rings are usually related to the outer Lindblad resonance, the inner rings to the inner 4/1-resonance and the nuclear rings to the inner Lindblad resonance. Alternatively, nuclear rings can be related to nuclear modes with higher pattern speed than that of the main bar.
2) The systems with very strong bars can lack nuclear and sometimes also inner rings. The nuclear rings can be absent also in systems where the bar rotates so fast that it does not have an inner Lindblad resonance. It is also possible that a starburst related to a nuclear rings consumes so much gas that the ring dissolves.
3) There are several ways to explain the absence of outer rings in barred galaxies. The age of the bar may be lesser than the timescale of outer ring formation, and thus systems with recently formed bars would naturally miss an outer ring. It is also possible, as suggested by Elmegreen et al. (1992), that interactions with other galaxies can destroy outer rings. However, the presence of the outer spiral mode with a lower pattern speed than that of the bar could not prevent outer ring formation. The situation is the same with the deceleration of the bar pattern speed and the corresponding change in the resonance positions: an existing ring reacts to the increasing outer Lindblad resonance radius by becoming larger.
4) Outer pseudorings can form also in the self-gravitating stellar component. These rings are much broader than the gaseous rings and can exhibit different morphology.
5) In cases with several modes near the OLR of the bar, the evolution can be almost cyclic: the morphology of the outer structure changes between different outer ring subclasses and occasionally there is not a clear ring or a pseudoring.
6) Systems where the orientations or the sizes of the rings are different to the normal values can be explained by assuming the presence of several modes with different pattern speeds. This is the case in one of our models, where the inner ring is misaligned with the bar. Such features do not form in models with a single analytical bar potential, but can be present in self-consistent simulations.
7) In addition to bars, rings can be formed by ovals. Also, spiral modes can produce rings if their lifetimes are long enough and their pattern speeds relatively constant. It is possible that at least part of the rings in non-barred galaxies are formed this way.
8) Some rings in non-barred galaxies can be formed by bars or ovals, which are later dissolved. In one of our simulations, this happened even without gas inflow or an interaction with another galaxy.
Our models have two major limitations: they are two-dimensional and the modelling of the gas component is very simplified. We intend to make improvements in both aspects in our forthcoming studies.
© European Southern Observatory (ESO) 2000
Online publication: October 24, 2000