6. SUMMARY OF NOTATION FOR RING AND LENS PHENOMENA
Tables 1 and 2 summarize the notation and classifications that we will use for resonance ring galaxies in the remainder of this review (see Buta 1995). The classifications in Table 2 are based on multicolor CCD images to be certain of the presence or absence of a nuclear ring. The list of galaxies includes virtually every conceivable combination of structures. It is noteworthy that even prototypical ``grand-design'' barred spirals like NGC 1300 can be included in such a list: it has a small nuclear ring inside the bar region. The list demonstrates that we can identify galaxies with up to four ring or pseudoring structures in co-existence. Doubled outer-ring galaxies appear to come in two types, the R1R2' type discussed in section 4.1, and the type illustrated by NGC 2273 (van Driel & Buta 1991; Figure 22) where the two features are clearly separated. No theory has yet explained this latter morphology, which is so far unique. Two outer-ringed ellipticals are also listed in Table 2. Corwin et al. (1985) classified IC 2006 as (R)SA0-, but the object is now recognized as an elliptical galaxy with an accretion ring of neutral gas that also displays some spiral character in deep B-band images (Schweizer et al. 1987a). It could be classified as an (R')E type, but the ring is not a resonance feature (it is counter-rotating relative to the main stellar body). The other ringed elliptical in Table 2, Hoag's object, is discussed further in section 16.
|R1||Type 1 OLR subclass outer ring|
|R1'||Type 1 OLR subclass outer pseudoring|
|R2'||Type 2 OLR subclass outer pseudoring|
TABLE II. Examples of Types with Ring Classifications
|Object||Revised Hubble Type|