5. GALAXY TYPES: STAGE, FAMILY, AND VARIETY

5.1. Elliptical and Spheroidal Galaxies

Elliptical galaxies are smooth, amorphous systems with a continuously declining brightness distribution and no breaks, inflections, zones, or structures, as well as no sign of a disk. Figure 4 shows some good examples. Because ellipticals are dominated by old stars and are relatively dust-free, they look much the same at different wavelengths. Hubble's subclassification of ellipticals according to apparent ellipticity (En, where n = 10(1-b/a), b/a being the apparent flattening) was useful but virtually no physical characteristics of ellipticals correlate with this parameter (Kormendy & Djorgovski 1989). The n value in the En classification is simply the projected ellipticity and not easily interpreted in terms of a true flattening without direct knowledge of the orientation of the symmetry planes. Luminous ellipticals are thought to be triaxial in structure with an anisotropic velocity dispersion tensor, while lower luminosity ellipticals are more isotropic oblate rotators (Davies et al. 1983). Studies of rotational to random kinetic energy (V / ) versus apparent flattening () show that massive ellipticals are slow anisotropic rotators. Ellipticals follow a fundamental plane relationship between the effective radius r_e of the light distribution, the central velocity dispersion ₀, and mean effective surface brightness <I_e> (see review by Kormendy & Djorgovski 1989). Dwarf elliptical galaxies may not follow the same relation as massive ellipticals; this is discussed by Ferguson & Binggeli (1994).

Figure 4. Examples of elliptical galaxies of different projected shapes. Type E galaxies are normal ellipticals with no structural details. From left to right the galaxies shown are NGC 1379, 3193, 5322, 1426, and 720. Type E+ galaxies are "late" ellipticals, which may include faint extended envelopes typical of large cluster ellipticals, or simple transition types to S0-. The examples shown are (left to right): NGC 1374, 4472, 4406, 4889, and 4623. All of these images are from the dVA (filters B, V, and g).

The lack of fundamental significance of Hubble's En classification led some authors to seek an alternative, more physically useful approach. Kormendy & Bender (1996; see also the review by Schweizer 1998) proposed a revision to Hubble's tuning fork handle that orders ellipticals according to their velocity anisotropy, since this is a significant determinant of E galaxy intrinsic shapes. Velocity anisotropy correlates with the deviations of E galaxy isophotes from pure elliptical shapes, measured by the parameter a₄/a, the relative amplitude of the cos4 Fourier term of these deviations. If this relative amplitude is positive, then the isophotes are pointy, disky ovals, while if negative, the isophotes are boxy ovals. A boxy elliptical is classified as E(b), while a disky elliptical is classified as E(d). The correlation with anisotropy is such that E(b) galaxies have less rotation on average and more velocity anistropy than E(d) galaxies.

The Kormendy & Bender proposed classification is shown in Figure 5, together with two exceptional examples that show the characteristic isophote shapes. The leftmost of these two, NGC 7029, is an unusually obvious boxy elliptical at larger radii. This boxiness is the basis for the classification E(b)5. However, NGC 7029 is not boxy throughout: it shows evidence of a small inner disk and hence is disky at small radii. This is not necessarily taken into account in the classification. The other image shown in Figure 5 is NGC 4697, a galaxy whose isophotes are visually disky. The idea with the Kormendy & Bender classification is that it is the disky ellipticals which connect to S0 galaxies, and not the boxy ellipticals. However, NGC 7029 demonstrates that diskiness and boxiness can be a function of radius, thus perhaps a smooth connection between E(b) and E(d) types [i.e., type E(b,d)] is possible and the order shown in the Kormendy & Bender revision to the Hubble tuning fork, with boxy Es blending into the disky Es, could be reasonable.

Figure 5. Revised classification of elliptical galaxies from Kormendy & Bender (1996), as schematically incorporated into Hubble's (1936) "tuning fork." At left are two examples of boxy and disky ellipticals: NGC 7029 (left, B-band) and NGC 4697 (JHKs composite, 2MASS image from NED).

Note that while the classical En classifications of ellipticals were designed by Hubble to be estimated by eye, this is not easily done for the E(b) and E(d) classifications, which are most favored to be seen only when the disk is nearly edge-on. Face-on Es with imbedded disks will not show disky isophotes. For example, NGC 7029 and 4697 are extreme cases where the isophotal deviations are obvious by eye. But for most E galaxies, the E(b)n and E(d)n classifications can only be judged reliably with measurements of the a₄ / a parameter.

The de Vaucouleurs classification of ellipticals includes a slightly more advanced type called E⁺, or "late" ellipticals. It was originally intended to describe "the first stage of the transition to the S0 class" (de Vaucouleurs 1959). Five examples of E⁺ galaxies are shown in the second row of Figure 4. Galaxies classified as E⁺ can be the most subtle S0s, but many of the E⁺ cases listed in RC3 are the brightest members of clusters that have shallow enough brightness profiles to appear to have an extended envelope (see section 10.6). Of the five E⁺ galaxies shown, only NGC 4623 (rare type E⁺7) seems consistent with de Vaucouleurs's original view. It is also a much lower luminosity system than the other four cases shown. While S0^- is the type most often confused with ellipticals in visual classification, the bin has a wide spread from the most obvious to the least obvious cases. Thus, a type like E⁺ is still useful for distinguishing transitions from E to S0 galaxies.

The photometric properties of ellipticals depend on luminosity. In terms of Sersic r^1/n profile fits, large, luminous ellipticals tend to have profiles described better by n 4, while smaller, lower luminosity ellipticals tend to have n < 4, with values as low as 1 (e. g., Caon, Capaccioli, & D'Onofrio 1993). Graham & Guzmán (2003) discuss the implications of this correlation on proposed dichotomies of elliptical galaxies (e.g., Kormendy 1985; see also Ferrarese et al. 2006). These studies received a major impetus from the massive photometric analysis of Virgo Cluster elliptical galaxies by Kormendy et al. (2009). Two issues were considered by these authors (Figure 6). The first was whether galaxies classified as dwarf ellipticals ("dE"; see section 15.2) in the Virgo Cluster really are the low luminosity extension of more massive, conventional ellipticals, or something different altogether. Based on parameter correlations, such as r_10%, the major axis radius of the isophote containing 10% of the total visual luminosity, versus µ_10%, the surface brightness of this isophote, Kormendy et al. show that even the most elliptical-like and luminous dE galaxies lie on a distinct sequence from normal elliptical galaxies, which tend to lie on a higher density sequence. In a graph of B-band central surface brightness versus absolute B-band magnitude, the dE galaxies lie in a region occupied by Magellanic irregular galaxies, suggesting a link between the groups and consistent with the earlier conclusions of Kormendy (1985). Kormendy et al. (2009) suggest reclassifying Binggeli, Sandage, & Tammann's (1985) "dE" galaxies and related objects (like dwarf S0, or dS0 galaxies) as "spheroidal" (Sph) galaxies, including the type "Sph,N", meaning "nucleated spheroidal" galaxy (section 15.2). Figure 6 shows several examples of Sph,N galaxies as compared with several genuine elliptical galaxies. The morphological appearance alone does not necessarily distinguish the two classes. The classification is physical, being based mainly on parameter correlations. Kormendy et al. suggest that Sph galaxies are formed from late-type systems by environmental effects and supernovae.

Figure 6. Illustrations of 6 early-type galaxies in the Virgo Cluster with photometric classifications from Kormendy et al. (2009): NGC 4472 (core E, MV = -23.2); NGC 4458 (coreless E, MV = -19.0); IC 798 (VCC 1440; `coreless E, MV = -16.9); NGC 4482 (nucleated spheroidal, MV = -18.4); IC 3470 (VCC 1431; nucleated spheroidal, MV = -17.4); IC 809 (VCC 1910; nucleated spheroidal, MV = -17.4). The images shown are all based on SDSS g-band single or mosaic images, and are in units of mag arcsec-2.

The second issue considered was the physical distinction between "core" elliptical galaxies, those where the surface brightness profile approaches either a constant level or a slightly sloped level with radius approaching zero, and "coreless" ellipticals (also known as "power law" Es) where the inner profile steepens with decreasing radius (Kormendy 1999). Kormendy et al. (2009) illustrated both types relative to a Sersic r^1/n fit to the outer regions of the luminosity profiles. In this representation, core Es are "missing light" relative to the fit while coreless Es have "extra light." The top row of Figure 6 shows one core E (NGC 4472) and two coreless Es [NGC 4458 and IC 798 (VCC 1440), the latter a low-luminosity dwarf]. The subtle distinctions are evident in these images, with NGC 4472 showing a soft center and NGC 4458 showing a strong center. The terminology for both types is mostly historical (Kormendy 1999) and somewhat counter to the visual impression (i.e., NGC 4472 lacks a bright core while NGC 4458 has one, yet the latter is technically coreless). Kormendy et al. show that core and coreless E galaxies have different Sersic indices, velocity dispersion anisotropy, isophote shapes, and rotational character, with the core Es being of the boxy type and the coreless Es of the disky type in Figure 5. The distinction may be tied to the number of mergers that formed the system.

5.2. S0 and Spiral Galaxies

The full classification of spiral and S0 galaxies involves the recognition of the stage, family, and variety. In de Vaucouleurs's classification approach, the implication for bars, inner rings, and stages is a continuum of forms (de Vaucouleurs 1959), so that there are no sharp edges to any category or "cell" apart from the obvious ones (for example, there are no galaxies less "barred" than a nonbarred galaxy, nor are there galaxies more ringed than those with a perfectly closed ring).

The classification of S0 galaxies depends on recognizing the presence of a disk and a bulge at minimum, and usually a lens as well, and no spiral arms. Examples are shown in Figure 7. The display of galaxy images in units of mag arcsec^-2 makes lenses especially easy to detect, as noted in the dVA. Even if a lens isn't obvious, a galaxy could still be an S0 if it shows evidence of an exponential disk. (Lenses are also not exclusive to S0s.) The "no spiral arms" characteristic is much stricter in the Hubble-Sandage classification than in the de Vaucouleurs interpretation, because varieties (r, rs, and s) are carried into the de Vaucouleurs classifications of S0s. This allows the possibility of a classification like SA(s)0^-, which would be very difficult to recognize. Bars enter in the classification of S0s in a similar manner as for spirals. Figure 7 shows mainly stage differences among nonbarred and barred S0s. The stage for S0s ranges from early (S0^-), to intermediate (S0°), and finally to late (S0⁺), in a succession of increasing detail. The earliest nonbarred S0s may be mistaken for elliptical galaxies on photographic images, and indeed Sandage & Bedke (1994) note cases where they believe an S0 galaxy has been misclassified as an elliptical by de Vaucouleurs in his reference catalogues (see also The Revised Shapley-Ames Catalogue, RSA, Sandage & Tammann 1981). This kind of misinterpretation is less likely for types S0° and S0⁺, because these will tend to show more obvious structure.

Figure 7. Examples of barred and nonbarred S0 galaxies of different stages from "early" (S0-), to "intermediate" (S0°), to "late" (S0+), including the transition stage to spirals, S0/a. The galaxies shown are (left to right): Row 1 - NGC 7192, 1411, 1553, and 7377; Row 2 - NGC 1387, 1533, 936, and 4596. All images are from the dVA (filters B and V).

The morphological distinction between E and S0 galaxies has been considered from a quantitative kinematic point of view by Emsellem et al. (2007). These authors argue that the division of early-type galaxies into E and S0 types is "contrived", and that it is more meaningful to divide them according to a quantitative kinematic parameter called _R, the specific angular momentum of the stellar component, which is derived from a two-dimensional velocity field obtained with the SAURON integral field spectrograph (Bacon et al. 2001). Based on this parameter, early-type galaxies are divided into slow and fast rotators, i. e., whether they are characterized by large-scale rotation or not. In a sample of 48 early-types, most were found to be fast rotators classified as a mix of E and S0 types, while the remainder were found to be slow rotators classified as Es. This kind of approach, which provides a more physical distinction among early-types, does not negate completely the value of the E and S0 subdivisions, but highlights again the persistent difficulty of distinguishing the earliest S0s from Es by morphology alone.

The transition type S0/a shows the beginnings of spiral structure. Two examples are included in Figure 7, one nonbarred and the other barred. Type S0/a is a well-defined stage characterized in the de Vaucouleurs 3D classification volume as having a high diversity in family and variety characteristics. The type received a negative characterization as the "garbage bin" of the Hubble sequence at one time because troublesome dusty irregulars, those originally classified as "Irr II" by Holmberg (1950) and later as "I0" by de Vaucouleurs, seemed to fit better in that part of the sequence. [In fact, de Vaucouleurs, de Vaucouleurs, & Corwin (1976) assigned the numerical stage index T = 0 to both S0/a and I0 galaxies.] However, this problem is only a problem at optical wavelengths. At longer wavelengths (e.g., 3.6 microns), types such as Irr II or I0 are less needed as they are defined mainly by dust (Buta et al. 2010a).

In general, the stage for spirals is based on the appearance of the spiral arms (degree of openness and resolution) and also on the relative prominence of the bulge or central concentration. These are the usual criteria originally applied by Hubble (1926, 1936). Figure 8 shows the stage sequence for spirals divided according to bar classification (SA, SAB, SB), and as modified and extended by de Vaucouleurs (1959) to include Sd and Sm types. Intermediate stages, such as Sab, Sbc, Scd, and Sdm, are shown in Figure 9. As noted by de Vaucouleurs (1963), these latter stages are almost as common as the basic ones.

Figure 8. Stage classifications for spirals, divided according to bar classifications into parallel sequences. The galaxies illustrated are (left to right): Row 1 - NGC 4378, 7042, 628, 7793, and IC 4182; Row 2 - NGC 7743, 210, 4535, 925, and IC 2574; Row 3 - NGC 4314, 1300, 3513, 4519, and 4618. All images are B-band from the dVA.

Figure 9. Sequences of stages intermediate between the main stages illustrated in Figure 8. The galaxies are (left to right): Row 1 - NGC 2196, 5194, 5457, and 4534; Row 2 - NGC 3368, 4303, 2835, and 4395; Row 3 - NGC 1398, 1365, 1073, and 4027. All images are B-band from the dVA, except for NGC 4534, which is SDSS g-band.

Stages Sdm and Sm are the most characteristically asymmetric stages, the latest spiral types along the de Vaucouleurs revised Hubble sequence. They are described in detail by de Vaucouleurs & Freeman (1972) and by Odewahn (1991). Sm is generally characterized by virtually no bulge and a single principle spiral arm. If a bar is present, it is usually not at the center of the disk isophotes, unlike what is normally seen in earlier type barred spirals. This leads to the concept of an offset barred galaxy. The single spiral arm emanates from one end of the bar. As noted by Freeman (1975), this is a basic and characteristic asymmetry of the mass distribution of Magellanic barred spirals. Sdm galaxies are similar, but may show a weaker or shorter second arm. In Figure 8, NGC 4618 is an especially good example of an SBm type (Odewahn 1991), while in Figure 9, NGC 4027 is illustrated as type SBdm.

An important issue regarding these galaxies is whether the optically offset bar is also offset from the dynamical rotation center of the disk. In a detailed HI study of the interacting galaxy pair NGC 4618 and 4625, Bush & Wilcots (2004) found very regular velocity fields and extended HI disks, but no strong offset of the rotation center from the center of the bar. This is similar to what Pence et al. (1988) found for the offset barred galaxy NGC 4027, based on optical Fabry-Perot interferometry. In contrast, both Magellanic Clouds, which are also offset barred galaxies, were found to have HI rotation centers significantly offset from the center of the bar (Kerr & de Vaucouleurs 1955).

In general, the application of Hubble's three spiral criteria allows consistent classification of spiral types. Nevertheless, sometimes the criteria are inconsistent. For example, small bulge Sa galaxies are described by Sandage (1961) and Sandage & Bedke (1994). Barred galaxies with nuclear rings can have spiral arms like those of an earlier Hubble type and very small bulges. In such conflicting cases, the emphasis is usually placed on the appearance of the arms. Also, while late-type Sdm and Sm galaxies are characteristically asymmetric, other types may be asymmetric as well. On average, the bulge-to-total luminosity ratio is related to Hubble type, but the result is sensitive to how galaxies are decomposed (e. g., Laurikainen et al. 2005). Asymmetry has been quantified by Conselice (1997).

The family classifications SA, SAB, and SB are purely visual estimates of bar strength, for both spirals and S0s. They are highlighted already in Figures 7-9, but the continuity of this characteristic is better illustrated in Figure 10, where de Vaucouleurs (1963) underline classifications (SAB and SAB) are also shown. An SA galaxy has no evident bar in general, although high inclination can cause a mistaken SA classification if a bar is highly foreshortened. Also, internal dust may obscure a bar (see, e. g., Eskridge et al. 2000). An SB galaxy should have a clear, well-defined bar. The intermediate bar classification SAB is one of the hallmarks of the de Vaucouleurs system, and is used to recognize galaxies having characteristics intermediate between barred and nonbarred galaxies. It is used for well-defined ovals or simply weaker-looking normal bars. The weakest primary bars are denoted SAB while the classification SAB is usually assigned to more classical bars that appear only somewhat weaker than conventional bars. Most of the time, galaxies which should be classified as SAB are simply classified as SB.

Variety is also treated as a continuous classification parameter (Figure 10, second row). A spiral galaxy having a completely closed or very nearly completely closed inner ring is denoted (r). The spiral arms usually break from the ring. If the spiral arms break directly from the central region or the ends of a bar, forming a continuously winding, open pattern, the variety is (s). The intermediate variety (rs) is also well-defined. Inner rings which appear broken or partial are in this category. The "dash-dot-dash in brackets" morphology: (-o-), where a bar with a bulge is bracketted by spiral arcs overshooting the bar axis, is very typical of variety (rs). The example of this shown in Figure 10 is NGC 4548. We use the notation rs to denote an inner ring made up of tightly wrapped spiral arms that do not quite close, while the notation rs is used for very open, barely recognizable, inner pseudorings. A good example of the former is NGC 3450, while an example of the latter is NGC 5371.

Figure 10. The continuity of family and variety characteristics among spiral galaxies, including underline classifications used by de Vaucouleurs (1963). The galaxies are (left to right): Row 1 - NGC 628, 2997, 4535, 3627, and 3513; Row 2 - NGC 2523, 3450, 4548, 5371, and 3507. All images are B-band from the dVA.

A spindle is a highly inclined disk galaxy. For blue-light images, usually an "sp" after the classification automatically implies considerable uncertainty in the interpretation, because family and variety are not easily distinguished when the inclination is high. Figure 11 shows, however, that stages can be judged reasonably reliably for edge-on galaxies. One important development in the classification of edge-on galaxies has been the ability to recognize edge-on bars through boxy/peanut and "X"-shapes. Boxy/peanut bulges in edge-on galaxies were proven to be bars seen edge-on from kinematic considerations (e. g., Kuijken & Merrifield 1995). This shape is evident in NGC 4425 (Row 1, column 4 of Figure 11; see also section 9).

Figure 11. Classification of edge-on galaxies by stage. The galaxies are (left to right): Row 1 - NGC 3115, 1596, 7332, 4425, and 5866; Row 2 - NGC 7814, 1055, 4217, 4010, and IC 2233. All images are from the dVA (B and V filters).

For spiral and S0 galaxies that are not too highly inclined (i.e., not spindles), once the stage, family, and variety are determined these are combined in the order family, variety, stage for a final full type. For example, NGC 1300 is of the family SB, variety (s), and stage b, thus its full type is SB(s)b. The S0⁺ galaxy NGC 4340 has both a bar and inner ring and its full type is SB(r)0⁺. The classification is flexible enough that if, for example, the family and variety of a galaxy cannot be reliably determined owing to high inclination, while the stage can still be assessed, then the symbols can be dropped and a type such as "Sb" or "S0" can still be noted.

5.3. Irregular Galaxies

Magellanic irregular galaxies represent the last normal stage of the de Vaucouleurs revised Hubble sequence. Several examples are shown in Figure 12. The objects illustrated in the top row are all examples of (s)-variety irregulars with bars or some trace of a bar. Nevertheless, not all Magellanic irregulars have bars. Irregulars of the lowest luminosities are usually classified simply as Im since the sophistication of structure needed to distinguish something like "family" may not exist for such galaxies.

Figure 12. Examples of irregular galaxies ranging in absolute blue magnitude from -14 to -18. The galaxies are (left to right): Row 1 - NGC 4449, 1569, 1156, DDO 50, and A2359-15 (WLM galaxy); Row 2 - IC 10, DDO 155, DDO 165, NGC 1705, NGC 5253. The "pec" stands for peculiar. All images are B-band from the dVA.

Irregular galaxies are important for their star formation characteristics. As noted by Hunter (1997), irregulars are similar to spirals in having both old and young stars, as well as dust, atomic, molecular, and ionized gas, but lack the spiral structure that might trigger star formation. Thus, they are useful laboratories for examining how star formation occurs in the absence of spiral arms.

Although irregulars are largely defined by a lack of well-organized structure like spiral arms, the two lower right galaxies in Figure 12 are not so disorganized looking and seem different from the other cases shown. NGC 5253 looks almost like a tilted S0 galaxy, yet it has no bulge at its center nor any obvious lenses. Instead, the central area is an irregular zone of active star formation. The central zone was interpreted by van den Bergh (1980a) as "fossil evidence" for a burst of star formation, possibly triggered by an interaction with neighboring M83. This is a case where the de Vaucouleurs classification of I0 seems reasonable: NGC 5253 is an early-type galaxy with a central starburst, probably the youngest and closest example known (Vanzi & Sauvage 2004). It is a Magellanic irregular galaxy imbedded in a smooth S0-like background known to have an early-type star spectrum. NGC 1705, also shown in Figure 12, is similar but has a super star cluster near the center and obvious peculiar filaments. It is classified as a blue compact dwarf by Gil de Paz et al. (2003). Both galaxies are classified as Amorphous by Sandage & Bedke (1994).