Apart from concentrating gas in the central regions of galaxies, as discussed in Section 2, bars also set up resonances which can act as focal points for the gas flow. As reviewed by, e.g., Shlosman (1999), gas concentrates there in limited radial ranges, where it can become gravitationally unstable and form stars. Rings in disk galaxies are mostly identified by their star formation, either by their blue colours or by H emission, and are intimately linked to the internal dynamics and the evolution of their hosts (see Buta & Combes 1996 for a comprehensive review on galactic rings).
Nuclear rings are those on scales of less than one to roughly two kiloparsec in radius. They are rather common, and occur in about 20% of nearby spiral galaxies (Knapen 2004b). They can be directly linked to inner Lindblad resonances (Knapen et al. 1995a; Heller & Shlosman 1996; Shlosman 1999), and are in fact found almost exclusively in barred galaxies (e.g., Buta & Combes 1996; Knapen 2004b; but see below for a counterexample). Individual gas clouds in a nuclear ring can undergo a Jeans-type collapse either spontaneously (Elmegreen 1994), or after compression by density waves set up by the bar (Knapen et al. 1995a; Ryder, Knapen, & Takamiya 2001). Nuclear rings are thus not only excellent tracers of massive star formation in starburst regions, but also of the dynamics of their host galaxies. The latter point is illustrated by Fig. 3, which shows that nuclear rings with a large size relative to their host galaxy can only occur in bars with a low gravitational torque, or: large rings cannot occur in strong bars. This confirms the results from earlier theory and modelling that the extent of the perpendicular x2 orbits needed to sustain the nuclear ring is limited as the bar gets stronger, i.e., as the x1 orbits become more elongated (see Knapen et al. 1995a; Heller & Shlosman 1996; Knapen, Pérez-Ramírez, & Laine 2002; Knapen 2004b), and graphically shows how intricately bars and nuclear rings are related.
Figure 3. Relative size (ring diameter divided by host galaxy diameter) for a sample of 15 nuclear rings as a function of the gravitational torque Qg, or strength, of the bar of its host galaxy. Data from Knapen, Pérez-Ramírez, & Laine (2002) and Knapen (2004b).
A small number of rings, or pseudo-rings, apparently occur in non-barred galaxies. Some of these hosts, although classified as non-barred from optical imaging in the major catalogues, are obviously barred when imaged in the near-IR (e.g., NGC 1068, Scoville et al. 1988, and NGC 4725, Shaw et al. 1993; Möllenhoff, Matthias, & Gerhard 1995). In other cases, a non-barred host galaxy may either have an oval distortion, or be undergoing the effects of an interaction with a companion galaxy. In either of these cases, the gravitational potential of the galaxy could be disturbed, and the non-axisymmetric potential could lead to ring formation, in much the same way as in the presence of a bar potential (Shlosman et al. 1989).
A nice example is that of NGC 278, a small, nearby and isolated spiral galaxy (vsys = 640 km s-1; D = 11.8 Mpc; D25 = 7.2 kpc; MB = - 18.8). Although classified as SAB(rs)b in the RC3, there is no evidence for the presence of a bar in this galaxy from either HST WFPC2 or ground-based NIR imaging (Knapen et al. 2004a). The optical disk of NGC 278 shows two distinct regions, an inner one with copious star formation and clear spiral arm structure, shown in Fig. 4, and an outer one (r > 27 arcsec or r > 1.5 kpc) which is almost completely featureless, of low surface brightness, and rather red. NGC 278 has a large HI disk, which is morphologically and kinematically disturbed, as seen from HI data (Knapen et al. 2004a). These disturbances suggest a recent minor merger with a small gas-rich galaxy, perhaps similar to a Magellanic cloud.
Figure 4. Real-colour image of the galaxy NGC 278 produced from archival F450W, F606W, and F814W HST images. Area shown is about 100 arcsec on a side, or about 5.7 kpc. The blue region, apparently made up of spiral arm fragments, indicates the nuclear ring, with a radius of 1.1 kpc. Image data from Knapen et al. (2004a).
The scale and morphology of the region of star formation in NGC 278 indicate that this is in fact a nuclear ring, albeit one with a much larger relative size with respect to its host galaxy than practically all other known nuclear rings (the absolute radius of the nuclear ring is about a kiloparsec, normal for nuclear rings). Knapen et al. (2004a) postulate that it is in fact the past interaction which has set up a non-axisymmetry in the gravitational potential, which in turn, in a way very similar to the action of a classic bar, leads to the formation of the nuclear ring. The case of NGC 278 illustrates how in apparently non-barred galaxies rings can be caused by departures from axisymmetry induced by interactions, but also shows how difficult it can be to uncover this: in the case of NGC 278 only through detailed HI observations.