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Astron. Astrophys. 362, 465-586 (2000)

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1. Introduction

Many galaxies have rings in their disks. These rings are probably the result of the internal evolution of their host systems, which are sometimes called ringed galaxies. When we speak of rings in this article, we mean this specific type of ring, not the so-called ring galaxies or the polar ring galaxies, which are likely created by interactions between galaxies (see e.g. Athanassoula & Bosma 1985).

Although rings are also found in non-barred galaxies, they are more common in barred systems (Buta & Combes 1996). It is generally accepted that the rings are located near resonances, which are induced by a bar or an oval component. It is thought that the gravitational torque of a rotating stellar bar drives interstellar gas into regions where the orbits of gas clouds are aligned either parallel or perpendicular to the bar major axis, and thus the net torque vanishes. In non-barred galaxies the rings may be related to resonances induced by a spiral mode or could be relics formed by a dissolved bar. It is also known that many apparently non-barred galaxies have a hidden bar or oval component that can be seen in near-IR (see e.g. Zaritsky & Lo 1986).

Rings are divided to outer, inner and nuclear rings based on their sizes relative to the bar. The major axes of the outer rings (R) and the pseudorings ([FORMULA]) are about twice that of the bar, the major axes of the inner rings (r) are usually filled by the bar and in the case of nuclear rings (nr), the major axes are about 1/10 of the bar major axis. There are two subclasses of outer rings and pseudorings; in the [FORMULA] and [FORMULA] subclass, the major axis of the ring is perpendicular to the bar and in the [FORMULA] subclass it is parallel to the bar. [FORMULA] pseudorings often have dimples near the ends of the bar. Complete detached outer rings can have similar features and thus also will be classified as [FORMULA], but usually it is impossible to apply subclasses to complete outer rings. There is also a combined outer ring morphology [FORMULA] (Buta & Crocker 1991; Buta 1995; Buta & Combes 1996).

The connection between inner rings and bars seems evident, although sometimes the bar ends before reaching the inner ring, and there are few galaxies where these structures are misaligned (Buta et al. 1995b). The inner rings are usually thought to be related to the inner 4/1-resonance of the bar. The situation with nuclear and outer rings is less clear. Although the nuclear rings are usually close to the deduced positions of the inner Lindblad resonance (ILR), they do not have clearly preferred position angles relative to the bar and it is possible that several of them are related to nuclear bars rather than the main bar component (Friedli & Martinet 1993; Buta & Crocker 1993). The shapes of the two subclasses of outer rings fit well to the shapes of the orbit families near the outer Lindblad resonance (OLR), but it has been stressed that the absence of outer rings in many barred galaxies also requires an explanation (see e.g. Sellwood & Wilkinson 1993). Furthermore, in many self-gravitating simulations, there is more than one mode present (Sellwood & Sparke 1988; Masset & Tagger 1997; Rautiainen & Salo 1999): the outer disk can be dominated by a mode that has a lower pattern speed than that of the bar. Intuitively, one would expect that in such situation, the outer ring would not form in the vicinity of the OLR of the bar. On the other, if the difference of pattern speeds of the outer disk and the bar is a typical situation, it is hard to understand why there are two subclasses of outer rings, differing by their orientation relative to the bar.

The previous studies of ring formation in barred galaxies (Schwarz 1981; Combes & Gerin 1985; Byrd et al. 1994; Piner et al. 1995) have used analytical two-dimensional rigidly rotating bar potential models. These studies are less realistic than N-body simulations, where the characteristics of the bar and the spiral arms are not arbitrary, but are determined by the initial stage of the systems. More realistic models can be constructed by deriving the potential from near-IR observations (e.g. Lindblad et al. 1996). This has been done for a few individual ringed galaxies, IC 4214 (Salo et al. 1999), ESO 565-11 (Buta et al. 1999b), and NGC 1433, NGC 3081 and NGC 6300 (Buta & Combes 2000). However, even these studies are inferior to N-body simulations because they model only the present stage of these systems, but do not reach their evolution. Moreover, these studies assume that a single mode dominates throughout the system. On the other hand, in most N-body studies on barred galaxies, only little attention has been paid to ring formation. Here our goal is to fill this gap by asking what modifications the N-body simulations (especially the possible presence of several simultaneous modes) can introduce to the ring formation process. We do not try to model any particular galaxy, but will frequently make qualitative comparisons with observations. In doing this, the Catalog of Southern Ringed Galaxies, CSRG (Buta 1995), has been most useful.

In the next section we describe the methods we have applied. We then present and analyse our simulation models. Finally, we discuss the relation between our simulations and observations.

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© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000