ELLIPTICAL GALAXY

An 'elliptical galaxy' is a type of galaxy in the Hubble sequence. Spherical to elliptical in shape, elliptical galaxies can range in size from hundreds of millions to over one trillion stars generally crowding at the center, while the outer regions contain scattered globular clusters. Most elliptical galaxies are composed of older low-mass stars, a sparse interstellar medium, and contain minimal star formation activity. Elliptical galaxies are believed to be approximately 70% of all galaxies. However, data may show discrepancies to this due the fact that Spiral galaxies are far more massive, and thus can be seen from distances where elliptical galaxies cannot.

Contents

General characteristics

Star formation

Sizes and shapes

Role in galactic evolution

Examples

See also

References

External links

General characteristics

Elliptical galaxies are characterized by several distinct physical properties. The motion of stars is dominated by random motion, unlike spiral galaxies, which have very little random motion and are dominated by rotation. Furthermore, there is very little interstellar matter, which translates to few young stars, few open star clusters, and rather old Population II stars. Also, larger elliptical galaxies typically have a system of globular clusters, indicating an old population. In addition, elliptical galaxies lack visible gas and dust.
Ellipticals and the bulges of disk galaxies have similar properties, and are generally regarded as the same physical phenomenon. Elliptical galaxies also tend to lie in the cores of 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. 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:
:
Thus for a spherical galaxy with ''a'' equal to ''b'', the number
is 0. The limit is about 7, which may indicate a physical process that prevents further flattening.^[1]
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).

Role in galactic 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.

Examples

★ M32

★ M49

★ M59

★ M60 (NGC 4649)

★ M87 (NGC 4486)

★ M89

★ M105 (NGC 3379)

★ M110

See also

★ Galaxy morphological classification

★ Lenticular galaxy

References

1. Galaxy Classification

External links

★ Elliptical Galaxies, SEDS Messier pages

★ Elliptical Galaxies