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).

Central concentration of light

The galaxy classification system proposed by Morgan (1958,1959a), which is sometimes referred to as the Yerkes system, is a one-dimensional scheme based on central concentration of light. Morgan arranged galaxies in a sequence a–f–g–k, with objects of type a having the weakest central concentration of light and those of type k having the strongest central concentration. The Yerkes system also recognizes the intermediates types af, fg and gk. Abraham et al. (1996b) have shown that it is possible to measure a central concentration index C, determined from measurements of the intensity-weighted second-order moments of a galaxy image, which is closely related to the central concentration classification of the Yerkes system.

Galaxies with Morgan type a tend to have early-type (A) spectra, whereas galaxies of type k mostly exhibit late-type (K) integrated spectra. This linkage between morphology and spectral type shows that the dominant stellar population in centrally concentrated galaxies is old, whereas objects with a low central concentration of light tend to have a strong young population component. Probably this correlation between central concentration and integrated spectral type is largely due to the fact that regions of high gas density will usually collapse at earlier times than is the case for low density regions. This is so because the collapse time-scale τ∝(Gρ)−½. As we shall see in Chapter 11 a puzzling exception is provided by the dwarf spheroidal galaxies, most of which are dominated by an old stellar population, even though they are of low density.

Luminosity effects on morphology

From a study of galaxies in the Virgo cluster Holmberg (1958, p. 69) noted that giant galaxies have a higher surface brightness than dwarfs. When the prints of the Palomar Sky Survey first became available in the late 1950s it was immediately obvious that this large, and very uniform, database of galaxy images enabled one to segregate lowluminosity dwarfs from the much more numerous galaxies of average luminosity. Based on inspection of the prints of the Palomar Sky Survey, van den Bergh (1959, 1966) was able to compile catalogs of 243 DDO dwarf galaxies north of δ= −27°. The entries in these catalogs showed that the distribution of such dwarfs on the sky is broadly similar to that of nearby giant galaxies. This conclusion was confirmed for Virgo dwarfs by Reaves (1956, 1967). Furthermore, observations in the Local Group clearly show that dwarfs cluster around giants. Van den Bergh's data also showed that (1) the fraction of all galaxies classified as irregular increases dramatically with decreasing luminosity, and (2) the fraction of all spirals that are barred is much lower among giants than it is among dwarfs. In a subsequent study van den Bergh (1960a,b,c) was able to show that both the surface brightnesses of spiral galaxies and their morphologies are functions of luminosity. Supergiant spirals were found to have long and well-developed ‘grand design’ spiral arms, whereas low-luminosity spirals tend to have poorly developed ‘scraggily’ spiral arms.

Cores and power-law profiles

Lauer et al. (1995) have used the Planetary Camera of the HST to image the central regions of 57 early-type galaxies. They found that the radial surface brightness profiles of most of these fall into two distinct classes: (1) galaxies that have cores, and (2) galaxies that exhibit power-law profiles that continue down to radii near the resolution limit. Of the galaxies observed by Lauer et al. 15 have cores and 30 exhibit power-law profiles. Among the galaxies that have been classified as having either cores or power-law profiles 21 are contained in A Revised Shapley–Ames Catalog of Bright Galaxies (Sandage & Tammann 1981). Since the statistics of objects in this catalog are better understood than those of the entire sample, only the nine galaxies with cores and the 12 having power-law profiles that are in the Shapley–Ames Catalog will be considered below. The most striking feature of these data (which has already been commented on by Lauer et al. and others) is that the galaxies with cores tend to be more luminous than those with power-law profiles. For the Shapley–Ames sub-sample a Kolmogorov–Smirnov test rejects the hypothesis that the galaxies with bulges were drawn from the same luminosity distribution as those having power-law profiles at the 97% confidence level.