3. COMPARISON OF RING CLASSIFICATIONS
The (r) and (s) varieties illustrated in Figure 7 are a well-established aspect of galaxy morphology that both the Hubble-Sandage and de Vaucouleurs types recognize as a part of normal galaxy morphology. However, since most observers choose samples for follow-up work on the basis of published catalog types, it is important to note that independent classifiers do not always agree on ring classifications. For example, some galaxies classified as (r)-variety by de Vaucouleurs (1963) or in the Third Reference Catalog of Bright Galaxies (RC3, de Vaucouleurs et al. 1991) are classified as (s)-variety by Sandage and Tammann (1981, 1987; hereafter RSA) and SB94. In the latter sources also, outer pseudorings are generally not recognized. The disagreement results because the spiral structure in many galaxies is very ring-like. In the Carnegie Atlas, strong ``near rings'' such as those in NGC 3081, 1433, 6782, and 7217 are noted but not formally recognized in the classification because the rings are made of tightly wound spiral structure. In the case of NGC 6782, SB94 state that ``the `ring' is two inner spiral arms that start in the nucleus [hence the subtype (s) rather than (r)] and nearly overlap after each has unwrapped by about half a turn''. Apparently, this effect is caused by a combination of the leading dust lanes in the bar and the tight spiral pattern that defines the inner ring in this case. In the case of NGC 3081, where the ``near ring'' has a very high contrast in the disk, SB94 state that the galaxy has ``one of the most complex morphologies of the RSA galaxies. There are rings within rings at the edges of disks within disks, as intricate a structure as in nesting, concentric Russian dolls.'' None of these rings, however, is recognized in the Carnegie classification of SBa(s). The de Vaucouleurs classification of the galaxy is instead (R)SAB(r)0/a.
Kostiuk (1975) compiled a list of 143 galaxies with ``outer ring-shaped structure'' as seen on the prints of the first Palomar Sky Survey. This list includes mostly true rings (not pseudorings), and she identified three types of rings: RS, rings which are some type of spiral surrounding a bright main body (examples: NGC 2859, 3945, and 4736); RH, rings with a weak halo beyond the ring (examples: NGC 4340, 4371, and 4608); and R, lacking a central nucleus (now known as the conventional ``ring'' galaxies). This study does not distinguish inner from outer rings or early from late-type galaxies, but the sample emphasizes early types by default.
Vorontsov-Velyaminov (1987) also classifies ring phenomena in galaxies in a manner very different from the Hubble-Sandage and de Vaucouleurs systems. In his descriptive classification, there is no recognition of subclasses of rings, such as inner, outer, or nuclear types. Instead, any galaxy with a prominent ring or pseudoring, whether luminous or dark (as in the absorption rings of S03 galaxies) is called a ring galaxy and classified as N;R in the Morphological Catalog of Galaxies (Vorontsov-Velyaminov et al. 1962-1968). Vorontsov-Velyaminov suggested that ring galaxies form a sequence parallel to and independent of spiral galaxies, and that there is a continuous transition of forms from pure amorphous disk galaxies, N;D, to ring galaxies. These are not allowed to have a bar or spiral arms. He suggests a smooth transition also between ring galaxies with a nucleus to those without one. Thus, there is no distinction between what the others would call ringed galaxies and what are commonly referred to as collisional ring galaxies (see Appleton & Struck-Marcell 1996), unless perhaps there is a bar. Polar ring galaxies are also referred to as ring galaxies by Vorontsov-Velyaminov, who describes the edge-on cases as ``lemons pierced by a needle.''
Vorontsov-Velyaminov also brought attention to ``double-stage'' spirals, where an inner set of spiral arms is largely independent of an outer set of arms. The inner arms may be more patchy than the outer ones and merit a later classification stage than the outer arms. Many of the galaxies where this ambiguity occurs have rings or pseudorings, such as NGC 3504, NGC 1808, and NGC 1433. In the Hubble Atlas and SB94, the inner set of arms determines the Hubble type.