2. HISTORICAL OVERVIEW
2.1. Inner Ring Structures
Ring-like structures in the luminosity distribution of galaxies have been known since photography was applied to nebular research in the late 19th and early 20th centuries. The discovery of rings parallels the discovery of spiral structure in the 19th century, because some ring and pseudoring structures in several nearby galaxies were bright enough to have been seen visually with the 1.8-m speculum reflector of Birr Castle Observatory and other telescopes (see Figure 2). The accuracy of these early observations was confirmed through photographic observations with the Lick Crossley reflector in 1898-1900 by Keeler (1908). However, it was Curtis (1918) who first brought attention to a special class of spiral ``nebulae'' where a ring or pseudoring and a bar were characteristic parts of the structure. Four examples, NGC 1300, NGC 1530, NGC 3351, and NGC 5921 were illustrated in his classic paper, a compendium of descriptions of 762 ``nebulae and clusters'', of which 513 were spirals. He called the ones with a ``band of matter extending diametrically across the inner parts'' and ``whorls'' forming a ``near perfect ring'', `` -type'' spirals, ``for lack of a better name.'' The objects Curtis first described are now recognized as ``barred spirals'', a term coined by Hubble (1926), and the near perfect ring as an ``inner ring.'' Although Curtis referred to them as -type, a term such as -type is actually more appropriate since the bar never exceeds the extent of the ring (see Figure 3). (1) Hubble (1926) would have used the term -type in his classification system, but Greek letters were inconvenient for catalogs in his day, so he chose the term ``barred spirals'' instead. Typically, the spiral structure of such a galaxy emerges from the inner ring, but Reynolds (1921) discovered two examples where only the shape was visible on his plates. Later, Hubble (1926, 1936) observed that the appearance of the ring in the -types was manifested by a tight spiral structure, which became more open as the spiral sequence progressed. In the -type spiral (NGC 1433) illustrated in Figure 3, the ring is a tight spiral pattern that closes around the bar ends.
The diversity of ringed galaxies became more apparent during later phases of Hubble's nebular research. In his discussion of Hubble's revision to the standard classification system, Sandage (1961) described in the Hubble Atlas of Galaxies an additional category of normal spirals where the arms begin tangential to the periphery of an ``internal'' ring. As with the -type spirals, these rings appeared to be linked to the spiral pattern in the sense that the rings become more open towards the later spiral stages (i.e., from Sa to Sc). Some of the first examples of this kind of ringed spiral were discovered by Shapley & Paraskevopoulos (1940), who called them ``plate spirals'' because of the image conveyed by a ring which tightly wraps the nucleus in a face-on view (i.e., the appearance of a central plate or disk). The original examples of ``plate spirals'' were NGC 6753 and the close pair NGC 6935 and NGC 6937, the latter now being known to contain a weak bar. Several of the examples of this kind of galaxy illustrated in the Hubble Atlas probably also have bars that are highly foreshortened (e.g. NGC 4274) or which are simply weak. Some ``plate spirals'' such as NGC 6902 (see Figure 4) appear perfectly nonbarred in blue light but show weak bars in near infrared images. Other nonbarred ringed galaxies, such as NGC 4736, have strong obvious ovals rather than conventional bars. Nevertheless, genuine examples of nonbarred ringed galaxies do exist (e.g., NGC 7217, see Figure 5; and NGC 4622, see Buta et al. 1992 and Section 17.1), although they are less common than the -types (see Section 9.1).
In a personal revision of the Hubble-Sandage classification system, de Vaucouleurs (1956, 1959a) considered the presence or absence of a bar as a ``family'' characteristic and the presence or absence of an inner ring as a ``variety'' characteristic. The continuous variation of these characteristics at a given stage (i.e., a galaxy can be ``weakly-barred'' or include a partial, broken ring) led him to construct a three-dimensional classification volume (Figure 6), a well-known cross-section of which (de Vaucouleurs & de Vaucouleurs 1964) is shown in Figure 7. The system is a generalization of a scheme illustrated by Sandage (1961, 1975), and in it pure inner rings (r) are simply the endpoint in a continuum of forms that becomes increasingly less ``ringed'' and more ``spiral'' (s) as one proceeds right to left in the classification volume. Those galaxies which are intermediate between these extremes are said to have ``pseudorings'', denoted by the combined symbol (rs). Figures 8, 9, 10 exhibit CCD images of galaxies covering the full range of families and varieties. Figure 7 also illustrates de Vaucouleurs' observations on how the morphology of the spiral structure emerging from the inner ring depends on family. In barred (SB) spirals, the spiral structure emerges from the ring as a global pattern, usually with two main arms. The ring itself is elongated along the bar axis, and has a relative size which is large compared to the extent of the outer structure (as in NGC 1433, Fig. 3). In nonbarred (SA) spirals, the spiral structure typically emerges from the inner ring as a multi-armed or a non-global pattern. The ring appears to be round, and has a small size compared to the extent of the outer structure [as in NGC 7217 (Fig. 5) and NGC 6753 (Figure 10, top)]. The mixed family (SAB) spirals have spiral patterns and rings whose characteristics are intermediate between these extremes. Of interest is the hybrid type, SAB(rs), where the inner pseudoring can have a characteristic hexagonal shape (as in NGC 4303, the ``model'' for the central sketch in Figure 7). The hybrid nature of this form causes the center of the classification volume to contain an excess number of objects compared to other subsections (section 9.1). On the basis of a variety of indicators, de Vaucouleurs (1970) suggested that our Galaxy is a hybrid ringed barred galaxy of type SAB(rs)bc.
There are many exceptions to the apparent ``rules'' in Figure 7. For example, IC 5240 (Fig. 8, top) is an SB(r)-type spiral where flocculent spiral structure breaks from the inner ring, not a global pattern, which is very unusual. Nevertheless, the inner ring and bar are strong, clear features. NGC 4622 (see section 17.1) and NGC 5364 (Sandage 1961) are examples of SA(r) or SA(rs)-type spirals where a strong global two-armed pattern breaks from the inner ring. In addition, the inner ring of NGC 4622 is very large compared to the extent of the spiral structure. These examples are discussed later in this review, and underscore a greater complexity of structure than can be illustrated in diagrams like Figure 7.
The parameter characterizing the third dimension in the classification volume is the stage T, which varies continuously in the sequence a, b, c, d, and m, with intermediate types ab, bc, cd, and dm. The stage expresses the development of spiral structure via the three principal classification criteria originally outlined by Hubble: the strength of the bulge, the degree of openness of the spiral structure, and the degree of resolution of the arms. Sandage (1961) noted that the pure ring form (r) parallels the pure spiral form (s) over the sequence Sa, SBa to Sc, SBc, but that at later stages the pure ring form is not common. He noted that most central rings in the Sc subclass are broken into several segments, characteristic of the (rs), pseudo-ringed variety. From a re-classification of 1500 bright galaxies taken mostly from the well-known catalog of Shapley & Ames (1932), de Vaucouleurs (1963) showed that the relative frequency of the pure ring variety (r) is highest near stage Sab, accounting for nearly 40% of the varieties observed at that stage, and that the phenomenon considerably weakened or became absent in the later spiral stages. Two late-type inner-ringed galaxies are shown in Figure 11.
Rings were also found in these early studies to be present in lenticular (S0) galaxies. In Hubble's original (1926) classification system, ringed, barred lenticulars were misclassified as SBa systems because the S0 category had not been fully recognized at the time. Sandage (1961; and more recently Sandage & Bedke 1994 = SB94) described Hubble's final classification of lenticulars. In nonbarred lenticulars, a central lens is surrounded by a faint envelope. A stage sequence (S01 -> S02 -> S03) is identified whereby the envelope and lens develop from a smooth luminosity distribution in early stages to a profile perturbed by a narrow, circular absorption ring. The galaxy NGC 4429, shown in Figure 12, bottom, has such an absorption ring. The region outside the absorption ring appears enhanced, giving the impression of an internal ring similar to those observed in spirals. Sandage did not associate these rings directly with the (r)-subtype, but de Vaucouleurs did classify them as inner rings. (The second, larger ring-like feature in NGC 4429 was classified by de Vaucouleurs as an outer ring.) Sandage noted a gradual transition whereby the absorption ring in the later lenticular stage begins to deviate from circular patterns and take on a tightly wound spiral form as the stage sequence progresses. In barred lenticulars (and many nonbarred ones as well), rings are often found at the edge of a lens, which also develops along the stage sequence into spiral arms. In fact, Sandage noted that the structural features of barred galaxies can be characterized by ``a sequence in which a simple pattern of bar and ring develops into a pattern of bar and spiral arms.''
The classification of lenticulars in the Hubble Atlas and in SB94 does not recognize varieties because S0 galaxies, by definition, do not have spiral structure. However, de Vaucouleurs adopted a different interpretation and recognized family and variety for lenticulars in the same manner as for spirals, i.e., as continuous properties. Ringed, pseudoringed, and non-ringed varieties at each family are recognized in an analogous manner to spirals. The principal difference with spirals is a notable deficiency of the hybrid type, SAB(rs)0, which de Vaucouleurs (1963) attributed to difficulty of classification. The strongest inner rings in lenticulars are characteristic of the ``late'' de Vaucouleurs lenticular type S(r)0+. NGC 4553 (Fig. 12, left) and NGC 7187 (Buta 1990b, Fig. 13) are two excellent examples of late, nonbarred lenticulars. NGC 4429 in Figure 12 is an unusual case because its ``outer ring'', which is actually an intermediate spiral pseudoring zone, bears an uncanny resemblance to the inner hexagonal zone of NGC 7020 (see Fig. 1). The significance of this similarity is not yet clear.