Galaxies are like people. The better you get to know them the more peculiar they often seem to become. Each individual galaxy may be thought of as representing a deviation from some underlying ideal type. ‘Classical morphology is useful because it succeeds to some extent in distinguishing galaxies which are physically different’ (Kormendy 1982, p. 125). It is the task of a galaxy morphologist (1) to recognize the archetype to which a galaxy belongs, and (2) to organize these archetypes of galaxies into a simple scheme that might eventually be interpreted in terms of galactic evolution. This review is mainly devoted to the morphology and classification of normal galaxies, which may be regarded as objects that are in the ‘ground state’ (Ozernoy 1974). Galaxies in excited states, such as quasars and Seyfert galaxies, will not be discussed in detail. Furthermore no mention will be made of purely descriptive classification systems such as those of Wolf (1908) and of Vorontsov-Velyaminov & Krasnogorskaja (1962). For a more detailed discussion of such systems the reader is referred to Sandage (1975). An Atlas of Peculiar Galaxies, that appear to fall outside the range of morphological types that are usually encountered among galaxies, has been published by Arp (1966). The vast majority of objects pictured in Arp's atlas appear peculiar because they are interacting (or have recently interacted) with their companions. However, some objects in Arp's catalog are dwarfs that are not peculiar at all (e.g. Atlas Nos. 2, 3, 4 and 5).

Butcher & Oemler (1978) discovered that distant rich clusters of galaxies contain more blue spiral galaxies than do similarly rich nearby clusters. This observation provided the first direct evidence for the evolution of galaxy morphology, i.e. that distant galaxies are (from an evolutionary point of view) younger than nearby galaxies. The limited resolution of ground-based telescopes made it difficult to follow this discovery up with more detailed studies of the evolution of galactic structure and morphology with increasing look-back time.

Galaxies viewed at large look-back times

A dramatic improvement of our ability to study distant galaxies is now provided by the HST. Abraham et al. (1996b) find that galaxies in the Medium Deep Survey (Driver, Windhorst & Griffiths 1995), which are typically located at z∼0.5, are basically still quite similar to those in the vicinity of the Milky Way, although the fraction of interacting galaxies (and objects that do not fit naturally within the Hubble classification scheme) is enhanced. Our deepest view into the past is provided by the HST observations in the Hubble Deep Field (Williams et al. 1996). These data are based on observations extending over 150 orbits in four colors of a field in Ursa Major. They provide images of 290 objects with 21<I<25. Abraham et al. (1996a) find that the fraction of asymmetrical and distorted galaxies is larger in the Hubble Deep Field than it is in the Medium Deep Survey.

White dwarfs

White dwarfs have low luminosities (∼ 10-4 - 1L⊙) but their photospheric temperatures (T ≃ 6000 – 2 × 104 K) are comparable to those of main-sequence stars. It follows that the radii of white dwarfs are small (≃ 109 cm). The masses and radii of several white dwarfs in binary systems have been determined. The white dwarfs Sirius B and 40 Eri B are known to have masses and radii equal to (M = 1.05 M⊙, R = 0.0074 R⊙) and (M = 0.48 M⊙, R = 0.0124 R⊙) respectively. Because the central densities of white dwarfs are high (≃ 106-109 g cm-3) and the temperatures of their isothermal cores relatively low (∼ 107 K), electrons are completely degenerate except in thin surface layers.

The structures of white dwarfs are determined by the equations of hydrostatic equilibrium (Equation (2.27)), mass conservation (Equation (2.28)) and the equation of state of an electron-degenerate gas described in Section 3.2. Unlike main-sequence stars the radii of white dwarfs decrease as a function of increasing mass and in addition there exists an upper limit to the mass of a white dwarf. This mass limit, which is known as the Chandrasekhar mass limit, depends on the electron molecular weight because the electron pressure increases as the number density of electrons increases. The calculated white dwarf mass limit Mc for uniform electron molecular weight μe is

Since most white dwarfs consist primarily of fully ionized 4He, 12C and 16O their electron molecular weight is μe = 2.

Observations of spiral arm types

From inspection of photographs of spiral galaxies Reynolds (1925) noted that some galaxies had ‘massive’ arms, whereas others exhibited ‘filamentous’ spiral structure. Another early attempt to classify galaxies on the basis of arm morphology was made by Danver (1942). More recently Elmegreen & Elmegreen (1982, 1987) have devised a twelve-stage classification system for spiral arms. These classifications range from Type 1 ‘flocculent’ arms, which are ragged, patchy, or chaotic to Type 12 ‘grand design’ arms, which are long, symmetrical, sharply defined, and dominate the appearance of the spiral galaxy in which they occur. After excluding barred spirals Elmegreen & Elmegreen (1982) find that 32 ± 10% of isolated objects exhibit well-developed spiral structure, compared to 67 ± 6% of members of binary pairs or groups. These results show that the formation of ‘grand design’ spiral structure is strongly favored by tidal interactions. Not unexpectedly Elmegreen & Elmegreen find a significant correlation between their spiral arm classification types and the luminosity classes of van den Bergh (1960a,b,c). Galaxies with patchy, fragmentary arms of Type 1 are all of low luminosity, whereas spirals with grand design spiral arms of Type 12 are, without exception, objects of high luminosity. The fact that spirals with very late Hubble types (Sd, Sdm, Sm) all have chaotic fragmentary arms of Types 1 and 2 is, no doubt, due to the low intrinsic luminosities of many very late-type spirals.

Introduction

It is surprising that Hubble (1936, p. 55, pp. 79–81) makes only a few passing references to the fact that early-type (E–S0–Sa) galaxies predominate in rich clusters, whereas the field is dominated by galaxies of late type (Sc–Ir). Spitzer & Baade (1951) were the first to emphasize the physical importance of the fact that the frequency of S0 galaxies is greatest in rich clusters of galaxies. Van den Bergh (1962) subsequently used the difference between the galactic populations in rich clusters and in the field to show that rich clusters must be stable over periods comparable to the age of the Universe. In particular the difference in the galactic populations of clusters and field provided a powerful argument against the tentative speculation by Hubble (1936, p. 81) ‘that the disintegration of clusters may populate the general field.’ The possible physical significance of the relation between galaxy morphology and environmental density was first discussed in great detail by Dressier (1980), who stressed that elliptical galaxies are most frequent in the regions of highest density, whereas late-type spirals predominate in low-density regions. A some-what different approach was taken by Whitmore & Gilmore (1991) who found that galaxy morphology was strongly correlated with distance from the cluster center. It is, of course, difficult to disentangle these effects because local density and distance from the cluster center are closely correlated. Sanromà & Salvador-Solé (1990) found that galaxy morphology does not appear to be affected by sub-clumpings within rich clusters.

Description of the system

In an attempt to accommodate the entire range of morphological characteristics of galaxies de Vaucouleurs (1959a) introduced a three-dimensional classification scheme which is illustrated in Figure 5. The main axis of this classification system is the sequence E–S0–Sa–Sb–Sc–Sd–Sm–Im, where the index m refers to magellanic, i.e. resembling the Magellanic clouds. Finer sub-divisions may be provided by distinguishing between E, E+, S0−, S0, and S0+, in which the minus superscript denotes early (= smooth) and the plus superscript indicates late (= patchy). In Figure 5 the second dimension is, as in the Hubble tuning fork diagram, provided by differentiating between galaxies with no bars (SA), those with weak bars (SAB) and those with strong bars (SB). Finally a third dimension is provided by distinguishing between objects that exhibit rings r, intermediate features rs and pure spiral arms s. De Vaucouleurs (1959a) notes that the distinction between his A and B families and between his r and s varieties is most clearly marked at the transition stage S0/a and vanishes between E and S0, and between Sm and Im. The position of a galaxy along the main axis from E to Im in Figure 5 correlates strongly with integrated color, and hence with the mean age of the stellar population. The A and B families do not differ systematically in color and hence, presumably, contain populations of comparable ages.

Ground state and excited galaxies

Excited galaxies (see Figure 26), i.e. objects that are not in the ‘ground state,’ fall into two broad classes: (1) diffuse galaxies with a high rate of star formation, such as starburst galaxies, Markarian galaxies, amorphous galaxies, Haro galaxies, intergalactic H II regions; and (2) objects with active nuclei, such as quasars, BL Lac objects (= blazars) and Seyfert galaxies. Most quasars and galaxies with active nuclei are located at large distances. This makes it difficult to study their morphology in detail. The images of luminous IRAS sources, including Seyfert 1, Seyfert 2, LINER (= low ionization nuclear emission-line region) and QSO parent galaxies by Hutchings & Neff (1991) suggest that many of them have a highly disturbed morphology including (what appear to be) tidal tails. Bahcall et al. (1996) show that the host galaxies of nearby quasars are frequently disturbed and interacting objects.

Active nuclei of galaxies can be fuelled by inward transport of gas (Morris & Serabyn 1996) via any one of the following processes:

• Gas may lose angular momentum to a stellar bar or oval disk by gravitational torques. Gas orbiting in a bar potential will have non-circular orbits resulting in collisions that produce shocks, which in turn result in loss of energy and angular momentum. The effect of spiral density waves is similar to that of bars, provided that there is no inner Lindblad resonance.

• […]

The image of a galaxy can be characterized in an entirely objective and non-controversial way by (1) its total integrated magnitude (usually the Holmberg (1958) magnitude based on the total luminosity inside isophotes of 26.5 and 26.0 mag arcsec−2 in the photographic and photo visual regions, respectively), (2) its integrated colors, U–B, B–V, V–R etc., (3) its isophotal radius r, in which r=(a×b)0.5, and where a and b are the semi-major and semi-minor axes to some specific isophote, (4) its effective radius re, defined as the radius within which half of the total galaxy light is emitted in projection, and (5) for many (but not all) galaxies the disk and bulge scale-lengths. More detailed characterizations, such as those provided by the Hubble, de Vaucouleurs and DDO type, are much more difficult (or perhaps impossible) to carry out in an entirely objective fashion.

Inspection of the images of the SRC Southern Sky Survey on IIIaJ emulsion (van den Bergh 1989b) show that it is just barely possible to recognize grand design spiral galaxies at redshifts of up to about 1×104 km s−1. The images of such objects have diameters of ∼0.5 mm (corresponding to 34') and contain ∼1×103 picture elements. Experience shows that the images of galaxies with redshifts of 1000–2000 km s−1, which typically contain 1×104−1×105 picture elements, can be classified with confidence on plates obtained with the SRC Schmidt telescope.

Selection effects

The disks of galaxies on the Hubble sequence Sa–Sb–Sc have central surface brightnesses that appear to fall in a rather narrow range (Freeman 1970). Zwicky (1957, p. 113) and Disney (1976) were among the first to emphasize the fact that this might be the result of a selection effect which is due to the difficulty in discovering galaxies of very low surface brightness. The fact that dwarf spheroidal galaxies, which are now known to be the most common type of extragalactic objects, were not discovered until the 1930s (Shapley 1939) supports this notion. It was originally thought (e.g. van den Bergh 1959) that all galaxies with a low surface brightness were early or late-type dwarfs. However, radial velocity observations by Fisher & Tully (1975) showed that some galaxies with low surface brightnesses are actually quite large and luminous. This effect is clearly shown in Figure 24 which compares the surface brightnesses of the disks of normal and of low-luminosity galaxies. An interesting feature of Figure 24 (see also Figure 1 of Bothun, Impey & McGaugh (1997)), which is presently not well understood, is that disk galaxies with surface brightnesses that are significantly higher than those of normal spirals do not appear to exist. This is shown most clearly in Courteau (1996b), who finds that there is a rather well-defined upper cut-off at a red central surface brightness of ∼17.5 mag arcsec−2.

The Hubble tuning fork diagram

The Hubble classification system recognizes three form families: ellipticals (E), spirals (S) and irregulars (Ir). The ellipticals are assigned an ellipticity ∈ defined as ∈=10(a –b)/a, in which a and b are the major and minor image diameters, respectively. Classification types for ellipticals range from E0, for objects that appear circular in projection, to E7 for the most highly flattened ellipticals. Spiral galaxies occur in two flavors – normal spirals (S), and barred spirals (SB). Within each of these there are three stages: Early-type galaxies of stage Sa/SBa have large nuclei and tightly coiled (and usually rather smooth) arms, objects in stage Sb/SBb have a more open spiral structure, and smaller central bulges. Late-type galaxies in stage Sc/SBc have small nuclear bulges and exhibit wide-open and rather patchy spiral arms. Finally, irregular galaxies have a patchy structure and exhibit no spiral arms. The original Hubble (1926) classification scheme was modified by Hubble (1936) who introduced a class of lenticular (S0) galaxies to span the chasm between spiral and elliptical galaxies (see Figure 2). The Hubble classification system is described and richly illustrated in The Hubble Atlas of Galaxies (Sandage 1961). Classifications for 1246 bright galaxies are given in A Revised Shapley–Ames Catalog of Bright Galaxies by Sandage & Tammann (1981). The Hubble/Sandage classification system reaches its ultimate form in The Carnegie Atlas of Galaxies (Sandage & Bedke 1994).