Types of galaxies
PRINCIPAL SCHEMES OF CLASSIFICATION
Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by Hubble in 1926. In Hubble's scheme, which is based on the optical appearance of galaxy images on photographic plates, galaxies are divided into three general classes: ellipticals, spirals, and irregulars. His basic definitions are as follows:Elliptical galaxies.
Galaxies of this class have smoothly varying brightnesses, with the degree of brightness steadily decreasing outward from the centre. They appear elliptical in shape, with lines of equal brightness made up of concentric and similar ellipses. These galaxies are nearly all of the same colour: they are somewhat redder than the Sun.Spiral galaxies.
These galaxies are conspicuous for their spiral-shaped arms, which emanate from or near the nucleus and gradually wind outward to the edge. There are usually two opposing arms arranged symmetrically around the centre. The nucleus of a spiral galaxy is a sharp-peaked area of smooth texture, which can be quite small or, in some cases, can make up the bulk of the galaxy. The arms are embedded in a thin disk of stars. Both the arms and the disk of a spiral system are blue in colour, whereas its central areas are red like an elliptical galaxy.Irregular galaxies.
Most representatives of this class consist of grainy, highly irregular assemblages of luminous areas. They have no noticeable symmetry nor obvious central nucleus, and they are generally bluer in colour than are the arms and disks of spiral galaxies. An extremely small number of them, however, are red and have a smooth, though nonsymmetrical, shape.Hubble subdivided these three classes into finer groups according to subtle differences in shape, as described in detail below. Other classification schemes similar to Hubble's follow this pattern but subdivide the galaxies differently. A notable example of one such system is that of Gerard de Vaucouleurs. This scheme, which has evolved considerably since its inception in 1959, includes a large number of codes for indicating different kinds of morphological characteristics visible in the images of galaxies. The major Hubble galaxy classes form the framework of de Vaucouleurs's scheme, and its subdivision includes different families, varieties, and stages, as shown in Table 1.
Examples of the de Vaucouleurs classification scheme are for galaxy M33, the Triangulum Nebula, which is classified as SA(s)cd, and the nearby small galaxy NGC 6822, classified as IB(s)m.
An entirely different kind of classification scheme is the luminosity classification developed in 1960 by Sidney van den Bergh. Based on morphological considerations, luminosity classes are assigned to individual galaxies within the Hubble classes. Those that are the most luminous are given a luminosity class of I, and the intrinsically faintest members of a class are assigned a V or VI, recalling the general approach of the luminosity class scheme used for stellar spectra . Thus a very luminous galaxy with open, resolved arms would be an Sc I galaxy, while a somewhat intrinsically fainter object with the same basic structure would be an Sc II or Sc III galaxy. To assign a luminosity class, a galaxy's image has to be compared with a set of standard images of galaxies for which distances are known and for which luminosity classes have been established by van den Bergh.
Classification schemes based on criteria other than optical appearance have been proposed. There is, for example, the Morgan scheme (proposed by W.W. Morgan), which combines information on the spectrum of a galaxy with its general shape. Here, a class is coded with a letter that indicates the spectral type of the galaxy in the blue (either as measured or as determined from the galaxy's bulge morphology, which correlates with the spectral type): e.g., a, af, f, fg, g, gk, k, for increasing dominance by cooler stars. The code then includes a capital letter to indicate general morphology--e.g., E, S, or I--in accordance with Hubble's general classes. This is followed by a number that indicates the overall optical shape of the image, with 0 representing a circular image and a 10 (never actually realized) standing for a linear, infinitely thin image. An example is the galaxy M31, the Andromeda Nebula, which is classified as kS5 in the Morgan system.
Systems that separate galaxies according to the character of their radio structure and the strength of their radio emissions also have been devised. For example, radio galaxies can be classified according to the following scheme:
g: galaxies with normal radio fluxes.
R: galaxies with strong radio emission. Many have distorted morphology, with evidence of explosive events or interactions with companions.
cD: galaxies with abnormally large, distended shapes, always found in the central areas of galaxy clusters and hypothesized to consist of merged galaxies.
S: Seyfert galaxies, originally recognized by the American astronomer Carl K. Seyfert from optical spectra. These objects have very bright nuclei with strong emission lines of hydrogen and other common elements, showing velocities of hundreds or thousands of kilometres per second. Most are radio sources.
N: galaxies with small, very bright nuclei and strong radio emission, probably similar to Seyfert galaxies but more distant.
Q: quasars, small, extremely luminous objects, many of which are strong radio sources. Quasars apparently are related to Seyfert and N galaxies but have such bright nuclei that the underlying galaxy can be detected only with great difficulty.
Although such schemes are sometimes used for special purposes, including, for example, certain kinds of statistical studies, the general scheme of Hubble in its updated form is the one most commonly used and so will be described in detail in the following section.
CLASSES OF GALAXIES
In The Hubble Atlas of Galaxies (1961), Allan R. Sandage drew on Hubble's notes and his own research on galaxy morphology to revise the Hubble classification scheme. Some of the features of this revised scheme are subject to argument because of the findings of very recent research, but its general features, especially the coding of types, remain viable. A description of the classes as defined by Sandage is given here, along with observations concerning needed refinements of some of the details.Elliptical galaxies.
 |
| M104, Sombrero Galaxy in Virgo, an elliptical galaxy surrounded by a disc of dust and gas. |
where I is the intensity of the light, Io is the central intensity, r is the radius, and a is a scale factor. The isophotal contours exhibited by an elliptical system are similar ellipses with a common orientation, each centred on its nucleus. No galaxy of this type is flatter than b/a = 0.3, with b and a the minor and major axes of the elliptical image, respectively. Ellipticals contain neither interstellar dust nor bright stars of spectral types 0 and B. Many, however, contain evidence of the presence of low-density gas in their nuclear regions. Ellipticals are red in colour, and their spectra indicate that their light comes mostly from old stars, especially evolved red giants.
Subclasses of elliptical galaxies are defined by their apparent shape, which is of course not necessarily their three-dimensional shape. The designation is En, where n is an integer defined by
A perfectly circular image will be an E0 galaxy, while a flatter object might be an E7 galaxy. (As explained above, elliptical galaxies are never flatter than this, so there are no E8, E9, or E10 galaxies.)
Although the above-cited criteria are generally accepted, current high-quality measurements have shown that some significant deviations exist. Most elliptical galaxies do not, for instance, exactly fit the intensity law formulated by Hubble; deviations are evident in their innermost parts and in their faint outer parts. Furthermore, many elliptical galaxies have slowly varying ellipticity, with the images being more circular in the central regions than in the outer parts. The major axes sometimes do not line up either, their position angles varying outward. Finally, astronomers have found that a few ellipticals do in fact have small numbers of luminous 0 and B stars as well as dust lanes.
Spiral galaxies.
 |
| Figure 1: Hubble's system of classification for galaxies (see text). |
S0 galaxies.
These systems exhibit some of the properties of both the ellipticals and the spirals and seem to be a bridge between these two more common galaxy types. Hubble introduced the S0 class long after his original classification scheme had been universally adopted largely because he noticed the dearth of highly flattened objects that otherwise had the properties of elliptical galaxies. Sandage's elaboration of the S0 class yielded the characteristics described here.S0 galaxies have a bright nucleus that is surrounded by a smooth, featureless bulge and a faint outer envelope. They are thin; statistical studies of the ratio of the apparent axes (seen projected onto the sky) indicate that they have intrinsic ratios of minor to major axes in the range 0.1 to 0.3. Their structure does not generally follow the luminosity law of elliptical galaxies, but it has a form more like that for spiral galaxies. Some S0 systems have a hint of structure in the envelope, either faintly discernible armlike discontinuities or narrow absorption lanes produced by interstellar dust. Several S0 galaxies are otherwise peculiar, and it is difficult to classify them with certainty. They can be thought of as peculiar Irr galaxies (i.e., Irr II galaxies [see below]) or simply as some of the 1 or 2 percent of galaxies that do not fit easily into the Hubble scheme. Among these are such galaxies as NGC 4753 that has irregular dust lanes across its image and NGC 128 that has a double, almost rectangular, bulge around a central nucleus. Another type of peculiar S0 is found in NGC 2685. This nebula in the constellation Ursa Major has an apparently edge-on disk galaxy at its centre, with surrounding hoops of gas, dust, and stars arranged in a plane that is at right angles to the apparent plane of the central object.
Sa galaxies.
These normal spirals have narrow, tightly wound arms, which usually are visible due to the presence of interstellar dust and in many cases bright stars as well. Most of them have a large, amorphous bulge in the centre, but there are some that violate this criterion, having a small nucleus around which is arranged an amorphous disk with superimposed faint arms. NGC 1302 is an example of the normal type of Sa galaxy, while NGC 4866 is representative of one with a small nucleus and arms consisting of thin dust lanes on a smooth disk.Sb galaxies.
This intermediate type of spiral typically has a medium-sized nucleus. Its arms are more widely spread than those of the Sa variety and appear less smooth. They contain stars, star clouds, and interstellar gas and dust. Sb galaxies show wide dispersions in details in terms of their shape. Hubble and Sandage observed, for example, that in certain Sb galaxies the arms emerge at the nucleus, which is often quite small. Other members of this subclass have arms that begin tangent to a bright, nearly circular ring, while still others reveal a small, bright spiral pattern inset into the nuclear bulge. In any of these cases, the spiral arms may be set at different pitch angles. (A pitch angle is defined as the angle between an arm and a circle centred on the nucleus and intersecting the arm.)Hubble and Sandage noted further deviations from the standard shape established for Sb galaxies. A few systems exhibit a chaotic dust pattern superimposed upon the tightly wound spiral arms. Some have smooth, thick arms of low surface brightness, frequently bounded on their inner edges with dust lanes. Finally, there are those with a large, smooth nuclear bulge from which the arms emanate, flowing outward tangent to the bulge and forming short arm segments. This is the most familiar type of Sb galaxy and is best exemplified by the giant Andromeda Galaxy.
Many of these variations in shape remain unexplained. Theoretical models of spiral galaxies based on a number of different premises can reproduce the basic Sb galaxy shape (see below The Milky Way Galaxy), but many of the deviations noted above are somewhat mysterious in origin and must await more detailed and realistic modeling of galactic dynamics.
Sc galaxies.
These galaxies characteristically have a very small nucleus and multiple spiral arms that are open, with relatively large pitch angles. The arms, moreover, are lumpy, containing as they do numerous irregularly distributed star clouds, stellar associations, star clusters, and gas clouds known as emission nebulas.As in the case of Sb galaxies, there are several recognizable subtypes among
the Sc systems. Sandage has cited six subdivisions: (1) galaxies, such as the
Whirlpool Nebula (M51), that have thin, branched arms that wind outward from a
tiny nucleus, usually extending out about 180
before branching into multiple segments; (2) systems with multiple arms that
start tangent to a bright ring centred on the nucleus; (3) those with arms that
are poorly defined and that span the entire image of the galaxy; (4) those with
a spiral pattern that cannot easily be traced and that are multiple and
punctuated with chaotic dust lanes; (5) those with thick, loose arms that are
not well defined--e.g., the nearby galaxy M33, the Triangulum Nebula; and
(6) transition types, which are almost so lacking in order that they could be
considered irregular galaxies.
Some classification schemes, such as that of de Vaucouleurs, give the last of the above-cited subtypes a class of its own, type Sd. It also has been found that some of the variations noted here for Sc galaxies are related to total luminosity. Galaxies of the fifth subtype, in particular, tend to be intrinsically faint, while those of the first subtype are among the most luminous spirals known. This correlation is part of the justification for the luminosity classification discussed earlier.
SB galaxies.
The luminosities, dimensions, spectra, and distributions of the barred spirals tend to be indistinguishable from those of normal spirals. The subclasses of SB systems exist in parallel sequence to those of the latter.There are SB0 galaxies that feature a large nuclear bulge surrounded by a disklike envelope across which runs a luminous, featureless bar. Some SB0 systems have short bars, while others have bars that extend across the entire visible image. Occasionally there is a ringlike feature external to the bar. SBa galaxies have bright, fairly large nuclear bulges and tightly wound, smooth spiral arms that emerge from the ends of the bar or from a circular ring external to the bar. SBb systems have a smooth bar as well as relatively smooth and continuous arms. In some galaxies of this type, the arms start at or near the ends of the bar, with conspicuous dust lanes along the inside of the bar that can be traced right up to the nucleus. Others have arms that start tangent to a ring external to the bar. In SBc galaxies, both the arms and the bar are highly resolved into star clouds and stellar associations. The arms are open in form and can start either at the ends of the bar or tangent to a ring.
Irregular galaxies.
Hubble recognized two types of irregular galaxies, Irr I and Irr II. The Irr I type is the most common of the irregular systems, and it seems to fall naturally on an extension of the spiral classes, beyond Sc, into galaxies with no discernible spiral structure. They are blue, highly resolved, and have little or no nucleus. The Irr II systems are rare objects. They include various kinds of chaotic galaxies for which there apparently are many different explanations. Table 2 compares various subgroups of this rather confusing assembly of objects.Some irregular galaxies, like spirals, are barred. They have a nearly central bar structure dominating an otherwise chaotic arrangement of material. The Large Magellanic Cloud is a well-known example. The Hubble system does not normally recognize this as a subtype, though the de Vaucouleurs classification scheme includes it and its related types as Im and IB (see Table 1).