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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.


TABLE I. Summary of notation for ring and lens phenomena
Feature symbol
nr nuclear ring
nr' nuclear pseudoring
r inner ring
rs inner pseudoring
R outer ring
R' outer pseudoring
R1 Type 1 OLR subclass outer ring
R1' Type 1 OLR subclass outer pseudoring
R2' Type 2 OLR subclass outer pseudoring
nl nuclear lens
l inner lens
rl inner ring/lens
L outer lens
RL outer ring/lens


TABLE II. Examples of Types with Ring Classifications
Object Revised Hubble Type
NGC 210 (R2')SAB(s)b
NGC 1300 (R')SB(s,nr)b
NGC 1317 (R')SAB(r'l,nr,nl)a
NGC 1326 (R1)SAB(r,nr)0/a
NGC 1433 (R1')SB(rs,nr)ab
NGC 1543 (R)SB(l)0°
NGC 1553 SA(rl)0o
NGC 2273 (RR)SB(rs)a
NGC 2983 (L)SB(rs)0+
NGC 3081 (R1R2')SAB(r,nr)0/a
NGC 4314 (R')SB(l,nrr')a
NGC 4321 SAB(s,nr')bc
NGC 4622 SA(r)ab
NGC 6753 (R')SA(nr)b
NGC 6782 (R1')SB(r,nr)0/a
NGC 7187 (R)SA(r)0+
NGC 7217 (R')SA(rs,nr)ab
IC 1438 (R1R2')SAB(r,nr)a
IC 1993 (R')SA(s)bc
IC 2006 (R')E1
IC 4290 (R')SB(r)a
IC 5240 SB(r)ab
ESO 287-56 (R1')SB(s)a
ESO 575-47 (R1')SB(rs)ab
ESO 577-3 (R2')SB(r)ab
Hoag's Object (R)E0


Figure 22. Illustration of NGC 2273, an unusual double-outer ring galaxy. This is a V-band CCD image with some inner contouring to show the bar.
NGC 2273

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