Definitions

Elliptical galaxy

An elliptical galaxy is a galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated to the famous Hubble space telescope) in his 1936 work “The Realm of the Nebulae” and, as such, forms part of the Hubble sequence. Elliptical galaxies have smooth, featureless light-profiles. They range in shape from nearly spherical to highly flattened ellipsoids and in size from hundreds of millions to over one trillion stars. In the outer regions, many stars are grouped into globular clusters. Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity. Elliptical galaxies are believed to make up approximately 10-15% of galaxies in the local Universe. They are preferentially found close to the centers of galaxy clusters and are less common in the early Universe.

General characteristics

Elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. The motion of stars in elliptical galaxies is predominantly radial, unlike the disks of spiral galaxies, which are dominated by rotation. Furthermore, there is very little interstellar matter (neither gas nor dust), which results in low rates of star formation, few open star clusters, and few young stars; rather, elliptical galaxies are dominated by old stellar populations, giving them red colours. Large elliptical galaxies typically have an extensive system of globular clusters.

The properties of elliptical galaxies and the bulges of disk galaxies are similar, suggesting that they are formed by the same physical processes, although this remains controversial. The luminosity profiles of both elliptical galaxies and bulges are well fit by de Vaucouleurs' law.

Elliptical galaxies are preferentially found in galaxy clusters and in compact groups of galaxies.

Star formation

This traditional portrait of elliptical galaxies paints them as galaxies where star formation has finished after the initial burst, leaving them to shine with only their aging stars. Very little star formation is thought to occur, because of the lack of gas, dust, and space. In general, they appear yellow-red, which is in contrast to the distinct blue tinge of a typical spiral galaxy, a colour emanating largely from the young, hot stars in its spiral arms.

Sizes and shapes

There is a wide range in size and mass for elliptical galaxies: as small as a tenth of a kiloparsec to over 100 kiloparsecs, and from 107 to nearly 1013 solar masses. This range is much broader for this galaxy type than for any other. The smallest, the Dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters. Most of these small galaxies may not be related to other ellipticals. The single largest known galaxy, M87 (which also goes by the NGC number 4486), is an elliptical.

It was once thought that the shape of ellipticals varied from spherical to highly elongated. The Hubble classification of elliptical galaxies ranges from E0 for those that are most spherical, to E7, which are long and thin in profile. It is now recognized that the vast majority of ellipticals are of middling thinness, and that the Hubble classifications are a result of the angle with which the galaxy is observed. The classification is typically determined by the ratio of the major (a) to the minor (b) axes of the galaxy's elliptical profile as follows:

$10 times \left(1 - frac\left\{b\right\}\left\{a\right\}\right)$
Thus for a spherical galaxy with a equal to b, the number is 0. The limit is about 7, which may be due to a bending instability that causes flatter galaxies to puff up.

There are two physical types of ellipticals; the "boxy" giant ellipticals, whose shapes result from random motion which is greater in some directions than in others (anisotropic random motion), and the "disky" normal and low luminosity ellipticals, which have nearly isotropic random velocities but are flattened due to rotation.

Dwarf elliptical galaxies are probably not true ellipticals at all; they have properties that are similar to those of irregulars and late spiral-type galaxies. Many astronomers now refer to them as "dwarf spheroidals" in recognition of this (note that this is still a topic of some controversy).

Evolution

Some recent observations have found young, blue star clusters inside a few elliptical galaxies along with other structures that can be explained by galactic collisions. Current thinking is that an elliptical galaxy is the result of a long process where two galaxies of comparable mass, of any type, collide and merge.

Such major galactic mergers are thought to have been common at early times, but may carry on more infrequently today. Minor galactic mergers involve two galaxies of very different masses, and are not limited to giant ellipticals. For example, our own Milky Way galaxy is known to be "digesting" a couple of small galaxies right now.

Every bright elliptical galaxy is believed to contain a supermassive black hole at its center. The mass of the black hole is tightly correlated with the mass of the galaxy, via the M-sigma relation. It is believed that black holes may play an important role in limiting the growth of elliptical galaxies in the early universe by inhibiting star formation.