2.3. Nuclear Ring Structures
The third type of ring structure, the nuclear ring, is the smallest of the three ring types. Some of the first examples were noted by Morgan (1958). In the presentation of his well-known spectroscopic classification system based on the forms and colors of galaxies, he isolated a class of galaxies with peculiar or complex nuclei. He described how some nuclei in spiral and barred galaxies are made up of multiple ``hotspots'', with type examples being NGC 1808, 3351, 4321, and 5248. [Note that the feature in NGC 4321 was already visible in a Lick Crossley photograph obtained by Keeler (1908), and that the feature in NGC 3351 was described by Curtis (1918).] Concurrently with this study, Sersic (1958) commented on the existence of an intriguing structure in the center of the SBb galaxy NGC 1097, where a ``central knot is surrounded by a chain of condensations forming a single spiral arm.'' Burbidge & Burbidge (1960) later observed this object and noted that the nucleus of the galaxy is a segmented annulus consisting of bright areas and probably dust.'' (The center of NGC 1097 had been noted long before these observations to have structure; see discussion in Shapley & Paraskevopoulos 1940). In the Hubble Atlas, Sandage (1961) brought attention to an intriguing case of a ``nuclear spiral'' in the center of the SBa galaxy NGC 4314, where the nucleus is characterized by a very small spiral pattern emerging from a knotty, ring-like zone centered on a small nucleus. He observed that this internal spiral is not connected in any way with the faint external arms of the galaxy.
Surprisingly, no clear examples of nuclear rings are described in the visual observation notes of W. Parsons and co-workers (Parsons 1926), although some may have been suspected. This may be explained in part by the fact that the rings are more prominent at blue than at visual wavelengths, by their location on the steep background light gradient of the bulge, and that some of the biggest and brightest examples are in the southern hemisphere and not accessible to the latitude of Birr Castle. The nuclear structures show a diverse variety of forms ranging from true rings of ``hotspots'' (e.g., NGC 6782 and ESO 565-11) to nuclear pseudorings (e.g., NGC 1097 and IC 4214) to almost pure nuclear spirals (e.g., NGC 4321, 1365, and 5383). Several examples of nuclear rings are illustrated with ground-based CCD images in Figure 15. The rings are small and generally not well-resolved by such observations, however. Barth et al. (1995, 1996) and Phillips et al. (1996) present HST images of several nuclear rings, which reveal the intricate dust lane patterns around and inside such features. Such high-resolution images reveal that even features classified as nuclear ``rings'' from the ground are in fact tightly wrapped spirals in many cases.
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The discovery of nuclear rings followed that of the previous two types of ring probably because of strong selection effects against detecting a very small, high surface brightness feature located in a region that is often overexposed on direct photographs. As the discussion implies, the phenomenon may be closely linked with the presence of a bar. This coincidence was quantified by Sersic & Pastoriza (1965, 1967), who found that in a carefully selected sample of 136 bright spirals over the whole sky, 20 contained the nuclear structure, and of these objects, all are either barred or weakly-barred. Unlike inner and outer rings, the nuclear ring phenomenon appears to be rare or absent in nonbarred galaxies, although this may be more a recognition difficulty than a real deficiency as noted by Buta and Crocker (1993; see section 5).