Sérsic & Pastoriza (1965, 1967) noticed thirty years ago that the circumnuclear regions of some barred galaxies have ``hot-spots'' of very intense star formation. It is now accepted that such hot-spots often delineate a ring-like structure surrounding the nucleus, generally coinciding with the location of the inner Lindblad resonance associated with some barred spirals. Gas torqued by the stellar bar loses angular momentum and flows inward, and if an inner Lindblad resonance is present, it accumulates in a tightly-wound, two-arm spiral or ring-like configuration at the position of the resonance (Elmegreen and Friedli, these proceedings). Large concentrations of molecular gas have been detected (e.g., Kenney et al. 1992; Kenney, these proceedings), and signs of intense star formation are often seen (e.g., Hummel, van der Hulst, & Keel 1987; Márquez & Moles 1993). Hence, it is fair to characterize some circumnuclear rings as ``starburst-like,'' even if the globally-averaged rate of star formation of the entire galaxy may not be that outstanding.
Figure 2. Four circumnuclear rings containing compact, young clusters from HST images taken at visual wavelengths. NGC 1097 and NGC 6951 are from Barth et al. (1995), and NGC 1019 and NGC 7469 are from Barth et al. (1996). The bright point source south of the nucleus is SN 1992bd.
HST images show that young clusters are found in great abundance in some circumnuclear rings (Fig. 2). In the case of NGC 1097, nearly 90 clusters were identified by Barth et al. (1995) in a ring-like structure of diameter ~ 1 kpc. Among those for which it was possible to perform reliable aperture photometry, the mean Rh 2.5 pc, in accord with the typical half-light radii seen in other SSCs. The clusters have a range of luminosities, the brightest having MV -14 mag, with a number between MV = -13 and -14 mag. Numerous clusters are also seen in the nuclear ring in NGC 6951, whose large-scale morphology appears very similar to the ring in NGC 1097. The greater distance of this galaxy, however, renders photometry of its clusters more difficult. Barth et al. (1995) find Rh 4 pc for several clusters and MV possibly as luminous as -15 mag, depending on the extinction (which is difficult or impossible to measure for individual clusters). Two other examples of nuclear rings from Barth et al. (1996) are also shown in Figure 2. The ring in NGC 1019 contains just a single bright cluster, whereas that of NGC 7469 has a large number. Note that the apparently strong correlation between circumnuclear rings and active galactic nuclei (all four examples have either Seyfert or LINER nuclei) is a result of a selection effect; the original samples from which these objects were selected largely targeted active nuclei.
The FOC UV imaging survey uncovered 5 additional circumnuclear rings richly populated with young clusters (Fig. 3). Although the rings in all but NGC 1079 were previously known from ground-based studies, what was not known was that the sites of star formation in the rings break up into discrete, compact units (clusters). In fact, Maoz et al. (1996a) find that the clusters constitute a significant fraction (15%-50%) of the total UV light in these objects. This estimate is a lower limit because there could be fainter clusters undetected at the current sensitivities of the snapshot images. The high detection rate of discrete sources implies that cluster formation is a significant, if not dominant, mode of star formation in these systems. Meurer et al. (1995) arrived at a similar conclusion from their study of 9 starburst systems. The luminosities of the clusters in the rings tend to be lower than those in the sample of Meurer et al. Strictly speaking, they do not qualify as SSCs, but it is clear that they are physically similar objects. The power-law luminosity function extrapolated from higher luminosities adequately fits the faint end of the distribution (Maoz et al. 1996a).
Figure 3. Four of the 5 circumnuclear rings from the FOC UV snapshot survey (Maoz et al. 1996a) containing compact, young clusters.