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The spiral structure of galaxies was discovered more than 170 years ago. The subtle patterns were first detected in 1845 with the world’s largest telescope at the time, the “Leviathan of Parsonstown” located in central Ireland. William Parsons, the Third Earl of Rosse, visually saw the spiral arms of the “Whirlpool Galaxy” M51 with his newly built 72-inch speculum metal reflector. In the parlance of 19th Century astronomy, M51 was called a “nebula”, not a galaxy, although the general view at the time was that most or all nebulae were distant systems of stars like the Milky Way (“Island Universe” hypothesis). Parsons built the Leviathan partly to test this idea. The discovery of spiral structure added mystique to the nebulae, and led to alternative ideas as to what the nebulae actually were. It would be nearly a century after Parsons’ discovery that any serious understanding of the nature of spiral structure would be achieved (section 1.11).

Spiral galaxies are generally two-component systems consisting of a bulge and a disk. Although at one time bulges were thought to be generally less flattened components than disks, it is now clear that bulges include a mix of dissimilar structures, such as spheroidal “classical” bulges, highly flattened “pseudobulges” (Kormendy and Kennicutt 2004) and “boxy/peanut” bulges, the latter thought to be due to edge-on views of bars (e.g. Lutticke and Dettmar 1999).

The classification of spirals is generally based on a rough correlation between the degree of central concentration and the character of the spiral arms. Hubble had noticed that galaxies with tightly wrapped, relatively smooth arms tended to have bright central bulges, while galaxies having open, relatively patchy spiral arms tended to have very small bulges. Hubble (1926) called the former cases Sa galaxies and the latter cases Sc galaxies, with Sb galaxies being intermediate between the two types. This observation set the stage for galaxy classification for nearly a century. However, the correlation works best for non-barred galaxies. It is poorer for barred galaxies that, even in cases with smooth, tightly wrapped arms, can have very small bulges. An example is NGC 3351, type SBb, which has a very small bulge in the center of a bright nuclear ring (Buta et al. 2007). Because of such inconsistencies, Sandage (1961) advocated basing spiral stage classifications (i.e. Sa, Sb, etc.) mainly on the appearance of the spiral arms.

Figure 1.9 shows a full CVRHS stage sequence for non-barred galaxies from stage S0° to stage Sm, that is, from the intermediate S0 stage (Figure 1.3) to the latest stage on the spiral sequence. The spiral sequence begins with the stage S0/a, which is considered to be a transition type between S0s and spiral galaxies. Type S0/a is a legitimate type in the sense that it is easily recognized and the continuity that de Vaucouleurs envisioned seems well represented by the type. Nevertheless, this apparent continuity does not necessarily imply that all S0s are correctly placed in the Hubble (1936) “tuning fork”. The stage generally begins with pseudorings made of tightly wrapped spiral structure as in NGC 809. In the CVRHS classification, stage S0/a is closer to S0 than to Sa, while S0/a is closer to Sa than to S0.

The sequence for non-barred galaxies in Figure 1.9 shows the rough correlation between central concentration and stage. Bulges are most prominent at stages Sbc and earlier, and are least prominent at stages Sc and later. The sequence shows well how arms are smooth at stage Sa and knotty, well-resolved, more open features at stage Sc. Intermediate stages are as well defined as regular stages: Sab galaxies often resemble Sa galaxies but with a greater degree of resolution into star-forming regions; Sbc galaxies typically have the bulge of an Sb galaxy in a disk with Sc arms; Scd is recognized as an Sc galaxy with only a trace of central concentration; and Sdm galaxies are typically bulge-less asymmetric systems with an offset bar and one spiral arm longer than the other. Similar underline stages (e.g. Sab. Scd) are used throughout the CVRHS sequence (de Vaucouleurs 1963).

Figure 1.10 shows the same kind of stage sequence for barred galaxies. Initially, Hubble (1926) believed that non-barred galaxies were the “normal” form of spirals, with perhaps maybe 20% of the spirals being barred. He nevertheless envisioned barred spirals as falling on a sequence parallel to that of non-barred spirals. In the Hubble Atlas of Galaxies, Sandage (1961) smoothly connects non-barred and barred S0s with the non-barred and barred spiral prongs, respectively, which is also true for the VRHS and the CVRHS systems. The same kinds of types are recognizable among barred spirals as among non-barred spirals. However, Figure 1.10 shows the small bulge effect in early-type barred spirals, an example being NGC 5610 whose smooth arms wrap into an outer pseudoring but whose bulge is no more prominent than that in an Sc galaxy.

Figure 1.9. A sequence of stages for non-barred galaxies in the VRHS/CVRHS system

The bars of spiral galaxies: As already noted in section 1.2, the CVRHS classification of bars utilizes five categories, SA, SAB, SAB, SAB and SB (de Vaucouleurs 1963), in a sequence of increasing visual bar strength (Figure 1.11). The classification is based on the length, contrast and axis ratio of the bar or bar-like feature. Although leading dust lanes are not a classification criterion for bars, such lanes are often seen in barred spirals of types S0/a to Sbc and could impact the apparent bar strength. Based on CVRHS classifications, the bar fraction is about 50% for SAB, SAB and SB cases, but increases to 67% if SAB is included (Buta et al. 2015; Buta 2019). The cosmological significance of the bar fraction is discussed by Sheth et al. (2008).

Figure 1.10. A sequence of stages for barred galaxies in the VRHS/CVRHS system

There are additional aspects of bars recognized in CVRHS classifications. Most bars are regular bars, the kinds of features seen in classic barred spirals like NGC 1300 or NGC 1365. Others are “ansae-type” bars, where the bar appears to have “handles” or enhanced spots at the ends (Martinez-Valpuesta et al. 2007). Several examples are shown in Figure 1.12, which are classified using the symbols SBa or SABa. In some cases, the appearance of ansae-type bars suggests a regular bar in the process of actual dissolution. On the other hand, Athanassoula et al. (2016) have recently used numerical simulations to show that ansae could form in the disk-shaped remnant of the merger of two spiral galaxies. Another aspect of bar classification comes from box/peanut bulge galaxies. A box/peanut bulge galaxy is generally an edge-on disk-shaped galaxy where the bulge has boxy isophotes and has the look of an “X” pattern crossing the center. One of these is shown in Figure 1.12 and several others are shown in Figure 1.13 (bottom row). The appearance of a boxy/peanut bulge can depend on whether the bar is viewed end-on or broadside-on. The X is believed to be a result of the line of sight view through vertical resonant bar orbits (e.g. Abbott et al. 2017). The classifications SBx and SABx are used for boxy/peanut bulges. In some cases, both a boxy/peanut bulge and ansae occur in the same system (as in NGC 5445; Figure 1.12). In such cases, the classification is SBax or SBxa.

Although the CVRHS bar family classification can be consistently applied, it is still a visual judgment and is not the most effective way of quantifying bar strength. It is also technically based on blue light images (the historical waveband of galaxy classification) where the appearance of the bar may be affected by dust and star formation. More quantitative approaches to bar strength include the maximum ellipticity in the bar region, the maximum relative m = 2 Fourier intensity amplitude A₂ = (I₂/I₀)_max and the ratio of the maximum tangential force to the mean radial force in the bar region, all based on near-infrared images (e.g. Combes and Sanders 1981; Buta and Block 2001; Buta 2012). Garcia-Gómez et al. (2017) describe the application of a two-dimensional Fourier transform technique to more reliably characterize the strengths of bars in disk galaxies.

Inner 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 , where the inner/nuclear variety is (r,nr,nb). Knapen (2005) determined that 21±5% of spiral galaxies have nuclear rings or related features.

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 (2019) as (R)SABa(rl,bl,nb)0⁺, meaning they are “late” S0s. A typical outer pseudoring is seen in NGC 5610, type (R′)SB(s)ab (Figure 1.10). Outer pseudorings are much more common than outer rings.

Among outer features are the resonant subclasses, that is, features that strongly resemble the kinds of resonance rings that form in numerical simulations of barred galaxies, specifically the models of Schwarz (1981) and Byrd et al. (1994). Figure 1.17 shows examples of the R₁, , and subclasses of outer rings and pseudorings. The way to recognize these features is described by Buta (2017b). The outer resonant subclasses constitute only a small fraction of outer features. Most disk-shaped galaxies do not have an outer feature, but among the ones that do, outer pseudorings of type R′ (excluding the resonant subclasses) are most abundant. The formation of rings has been reviewed in Buta and Combes (1996) and is discussed further in section 1.12.

Figure 1.16. Examples of different types of outer features. The feature type is in the upper right of each frame