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Types of Galaxies

Elliptical Galaxies Elliptical galaxies appear in the sky as luminous elliptical disks. Elliptical galaxies have more stars concentrated at their center than at their outer edges, but otherwise have little or no structure. The light distribution is smooth with the surface brightness decreasing smoothly outward from the center.

Elliptical Galaxy
Image Provided By Anglo-Australian Observatory

The actual light distribution of ellipticals is well represented as

log I(r) = C0 r-1/4

Where C0 is the central brightness, and I(r) is the brightness at radius r from the center. These galaxies are classified by means of the elongation of the projected image. That is, if a and b are the major and minor axes of the projected ellipse, then the ellipticity class, Ec,

Ec = 10 ( a - b ) / a

is a measure of the ellipticity. This quantity is observed to vary fairly smoothly from E0 to E7. Statistical studies indicate that the true shapes of the galaxies are uniformly distributed from E0 to E7. There is no indication of the presence of any dust in these galaxies.

cD Galaxies

This type of galaxy appears similar to the ellipticals, and they are sometimes referred to as giant ellipticals. The type was introduced by W. W. Morgan. The "c" refers to an old spectroscopic designation for supergiants, while the "D" simply stands for diffuse. The objects appear to have greatly extended envelopes and frequently exhibit structure and multiple nuclei near their centers. They may have sizes which are ten times larger than a normal elliptical.

A cD Galaxy
Image Provided by John Gretchen

cD galaxies are probably the result of two or more objects merging together, or of a larger object "swallowing" or cannibalizing smaller galaxies.

Irregular Galaxies

Irregular galaxies show no symmetrical or regular structure. Galaxies of type Irr I have resolved OB stars and HII regions. Irr II do not resolve into stars and are generally amorphous. Irregulars seem to contain a higher than usual concentration of dust and gas. Because of the dependence on resolution, it is impossible to distinguish between Irr I and Irr II for more distant objects.

Image of an Irregular Galaxy
NGC 55
Image Provided By Anglo-Australian Observatory

Hubble described this class as lacking both dominating nuclei and rotational symmetry.

In general, this class displays a lack of any organized structure.

The suffix p, for peculiar, should generally be reserved for galaxies with obvious structure that has been distorted by the tidal interaction with close companions.

Lenticular Galaxies

Lenticular galaxies (Type S0) are intermediate between the E7 ellipticals and the Sa spirals. They are flatter than E7s and have a thin disk as well as a spheroidal nuclear bulge. When they are seen edge on they sometimes have the shape of a convex lens, so they are also called lenticulars. The disk components have a light distribution that falls off more slowly than in the ellipticals.

I(r) = I0 e-ar

An S0 galaxy seen edge-on is very difficult to distinguish from an Sa seen edge-on. An S0 seen face-on is very difficult to distinguish from and E0. High quality images are necessary in order to discern the faint disk. Thus at increasing distances the identification of the S0s becomes increasingly more difficult.

Lenticular Galaxy
NGC 936
Image Provided By Sloan Digital Sky Survey

S0 galaxies are disk galaxies, like spirals, but they have far less dust and gas than a normal spiral. It is possible that S0s are early type spirals that have lost their dust and gas as a result of tidal interactions with other galaxies. It is even possible that S0s are early type spirals which have suffered collisions. One clue to the nature of lenticular galaxies is that they are preferentially found near the centers of rich clusters of galaxies, where the intracluster gas would strip the interstellar dust and gas from a galaxy as it falls through the cluster.

Ring Galaxies

There are galaxies known to have very large ring-shaped structures beyond the nucleus. Some of these galaxies look almost like the planet Saturn, with their large prominent ring surrounding the nucleus. Sometimes the nucleus is nearly absent, so the only structure apparent is a large ring. In either case, these galaxies are termed "ring galaxies" and have been classified as R (for "Ring"). If an underlying spiral structure is discernible, the galaxy might be classified as S(R) or SB(R). Note that the upper case R is used for large ring structures beyond the nucleus, while the lower case r is used for ring structures features within or near the nucleus.

Ring Galaxy
Hoag's Object
This image was produced by the Hubble Heritage Team (STScI/AURA) using data collected by the Hubble Heritage team (STScI/AURA) and Ray Lucas (STScI).

It has been suggested by some that ring galaxies are produced as a consequence of a galaxy-galaxy collision.

Dwarf Galaxies

Most of the galaxies with which we are familiar are bright and easily detected. However, the vast majority of galaxies in the universe are actually small and faint. They tend not to fit into the classification scheme as we have outlined thus far. The most common form for these galaxies appears to be elliptical, and since they generally contain little or no gas, they are as a class referred to as dwarf ellipticals, dE. In addition to being smaller than the bright elliptical galaxies, the dEs do not exhibit a bright nuclear region. Another classification used for dwarf galaxies is dIrr (for dwarf irregular).

A Dwarf Galaxy
Leo I
Image Provided by Anglo-Australian Observatory

De Vaucleures has proposed the term "compact" to designate the dwarf galaxies. Thus, one can encounter the designations cE, for compact ellipticals, and cI, for compact irregulars.

Since these objects are faint, they can only be detected if they are relatively nearby. This in itself can present a problem because some dwarf galaxies are so sparsely populated with stars that they are difficult to distinguish from Milky Way foreground stars. Nonetheless, given the large numbers of dwarf galaxies that we find in the local group and in other nearby groups, these are likely the most common types of galaxies in the universe.

Spiral Galaxies

Spiral galaxies are either ordinary (S or SA) or barred (SB). Both types have spiral shaped arms, with two arms generally placed symmetrically about the center of an axis of rotation. In the ordinary spirals the arms emerge from the nucleus, while in the barred spirals the arms emerge near the ends of a bar-like structure which passes through the center of the nucleus. Both types are classified according to how tightly the arms are wound, how patchy they are, and the relative size of the nucleus.

Spiral Galaxy
NGC 5236
Image Provided By Anglo-Austrailian Observatory

Ordinary spirals of type Sa have smooth ill-defined arms that are tightly wound around a large prominent nucleus. The arms are wound so tightly they are nearly circular. The intermediate Sb galaxies have more open arms which are often partly resolved into HII regions and stellar associations. The nuclei of Sc galaxies are usually quite small and the arms are well extended and resolved into HII regions and clumps of stars. An Sd galaxy displays virtually no nucleus at all.

Hubble described this sequence in the following manner:

The arms appear to build up at the expense of the nuclear regions and unwind as they grow; in the end the arms are wide open and the nuclei inconspicuous. Early in the series the arms begin to break up into condensations, the [breakup] commencing in the outer regions and working inwards until in the final stages it reaches the nucleus itself. The "breakup" referred to is the appearance of HII regions and bright blue supergiants.

More on Spiral Galaxies!

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This page was last modified on Wednesday 13th June 2012 @ 10:11am

Science Mission Directorate Universe Division

Responsible SSU Personnel:

Dr. Kevin McLin (mclin at universe dot sonoma dot edu)

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