varieties: The inner variety of any disk-shaped galaxy refers to the presence or absence of an inner ring (Sandage 1961). If an inner ring is present, the inner variety is (r). In a spiral, the spiral structure breaks from near the location of the inner ring. The inner variety is (s) if there is no inner ring and the spiral structure either winds all the way to near the center of the galaxy or breaks directly from the ends of a bar. In many galaxies, a partial inner ring made of tightly wrapped spiral structure is seen. As noted in section 1.2, such “pseudorings” are recognized by the symbols (rs) in the variety sequence: (s), (rs), (rs), (rs), (r), where the underlines denote the dominant characteristic. Examples of these morphologies are shown in the upper row in Figure 1.14.
Inner rings are most common in barred galaxies, but also appear in non-barred galaxies. Some non-barred galaxies with rings could be evolved remnants of an earlier barred phase, owing to the possibility that bars may dissolve in much less than a Hubble time due to a buildup of the central mass concentration (Norman et al. 1996).
Figure 1.11. A sequence of increasing apparent bar strength
Figure 1.12. Bars showing enhanced “handles”, or ansae. The features appear in spot, linear or curved form
Figure 1.13. Three edge-on galaxies showing boxy/peanut-type bulges
A different inner variety sequence is sometimes applicable. As noted in section 1.3, early-type galaxies often show inner lenses, which are features located in the same place where an inner ring would be seen. If a bar is present, the bar usually fills the inner lens in one dimension (Kormendy 1979). The symbol for an inner lens is (l) and that for an inner ring-lens is (rl). These are used in the sequence: (r), (rl), (rl), (rl), (l). Examples of these morphologies are shown in the bottom row of Figure 1.14. In some cases, the inner variety is r′l, meaning an inner pseudoring-lens. In an actual classification, the inner variety is in parentheses between the family and the stage [as in, e.g. SB(r)b, SAB(rs)cd, SA(l)0/a, etc.].
The relation between inner rings and inner lenses is unclear. One possibility is that an inner lens is a highly evolved inner ring. This might account for the existence of inner ring-lenses (rl), which appear to be low contrast inner rings. However, Kormendy (1979; see also Bournaud and Combes 2002; Gao et al. 2018) proposed another interpretation: that inner lenses represent dissolved bars. Bar dissolution is possible because the presence of a bar not only heats the disk component, but also causes resonance effects that force stars onto orbits that do not support the bar. An example of the latter is the formation of a nuclear bar, which is a small secondary bar that forms inside a primary bar. Such features are recognized with the symbol (nb) in the CVRHS classification system, and are often significantly misaligned with a primary bar if present.
An interesting aspect of inner rings and lenses is that the former are most common in barred galaxies, but the latter are most common in non-barred galaxies. A possible reason for this is that bar dissolution could leave behind a lens that was formerly the inner part of the bar, called a barlens (Laurikainen et al. 2013). A barlens [symbolized by (bl)] is generally the roundish, inner component of a bar that often is mistaken for a classical bulge. Examples are shown in Figure 1.18. Athanassoula (2016) interprets barlenses as the three-dimensional inner sections of bars that appear as boxy/peanut bulges in the edge-on view. The ends of the bar are much flatter than this inner section. In general, the boxy character of these inner sections is not very evident in the near face-on view. However, in some bars, an inner boxy zone is seen even in a lower inclination view (examples: NGC 7020, IC 4290, IC 5240; Buta et al. 2007).
Another interesting aspect of inner rings is that these features have a wide range of intrinsic shapes (deprojected minor-to-major axis ratio 0.5 to 1.0; Buta 2019) and are often regions of intense star formation. The distribution of star formation in inner rings is sensitive to this range: the more elongated the ring, the greater the concentration of HII regions around the major axis points (Crocker et al. 1996; Grouchy et al. 2010). The effect is especially evident in cuspy-shaped inner rings, of which NGC 6782 is the best example (Lin et al. 2008). It is also seen in NGC 3081 (Buta and Purcell 1998).
Figure 1.14. Examples of different inner varieties
Nuclear varieties: The nuclear variety of a disk-shaped galaxy refers to the presence of nuclear structure, usually in the form of a nuclear ring (nr), nuclear pseudoring (nr′), nuclear spiral (ns), nuclear bar (nb), nuclear lens (nl) or nuclear ring-lens (nrl). The features tend to be small and therefore are recognizable mainly in nearby galaxies. The features also have a wide range of linear diameters, from a few hundred pc to nearly 5 kpc (Comerón et al. 2010). In some cases, a nuclear ring is crossed by a nuclear bar. Figure 1.15 shows several examples of spiral galaxies having a nuclear ring. In a CVRHS classification, the nuclear variety appears with the inner variety. For example, NGC 3081 is classified as
Figure 1.15. Examples of spiral galaxies having a nuclear ring
Outer varieties: Just as inner varieties refer to inner rings and lenses mainly, outer varieties refer to outer rings and lenses. While a typical inner ring or pseudoring in a barred galaxy has about the same size as the bar, the typical outer ring is about twice the size of the bar. Outer features are typically more diffuse than inner features, and of lower surface brightness. The main types of outer features include outer rings (R), outer pseudorings (R′), outer lenses (L), outer ring-lenses (RL) and outer pseudoring-lenses (R′L). An example of each of these, including a doubled outer ring case (RR), is shown in Figure 1.16. In all cases, the outer feature classification is positioned ahead of the family classification, as in (RL)SA(l)0° or (R′)SB(rs)ab.
In spiral galaxies, outer features tend to be pseudorings or pseudoring lenses. The best-defined outer rings tend to be found in S0+ or S0/a cases. The galaxies NGC 2859 (Figure 1.16) and 3945 (Figure 1.14) are two of the best examples of outer rings. Both are classified by Buta
