Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars.
Elliptical galaxies are one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Nebulae”,[1] along with spiral and lenticular galaxies.
Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity. They are surrounded by large numbers of globular clusters. Elliptical galaxies are believed to make up approximately 10–15% of galaxies in the local Universe[2] but are not the dominant type of galaxy in the universe overall. They are preferentially found close to the centers of galaxy clusters[3] and are less common in the early Universe.
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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.[4]
The dynamical properties of elliptical galaxies and the bulges of disk galaxies are similar, [5] 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 Sersic's 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 Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is. The classification is determined by the ratio of the major (a) to the minor (b) axes of the galaxy's isophotes:
Thus for a spherical galaxy with a equal to b, the number is 0, and the Hubble type is E0. The limit is about E7, which is believed to be due to a bending instability that causes flatter galaxies to puff up. The most common shape is close to E3. Hubble recognized that his shape classification depends both on the intrinsic shape of the galaxy, as well as the angle with which the galaxy is observed. Hence, some galaxies with Hubble type E0 are actually elongated.
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 have properties that are intermediate between those of regular elliptical galaxies and globular clusters. Dwarf spheroidal galaxies appear to be a distinct class: their properties are more similar to those of irregulars and late spiral-type galaxies.
At the large end of the elliptical spectrum, there is further division, beyond Hubble classification. Beyond gE giant ellipticals, lies D-galaxies and cD-galaxies. These are similar to their smaller brethren, but more diffuse, with larger haloes. Some even appear more akin to lenticular galaxies.
Evolution
Current thinking is that an elliptical galaxy may be 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 "ingesting" a couple of small galaxies right now. The Milky Way galaxy, is also, depending upon an unknown radial component, on a collision course in 3-4 billion years with the Andromeda Galaxy. It has been theorized that an elliptical galaxy will result from a merger of the two spirals.
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.
Examples
- M32
- M49
- M59
- M60 (NGC 4649)
- M87 (NGC 4486)
- M89
- M105 (NGC 3379)
- Maffei 1, the closest giant elliptical galaxy.
See also
- Firehose instability
- Galaxy color-magnitude diagram
- Galaxy morphological classification
- Hubble sequence
- Lenticular galaxy
- M-sigma relation
- Osipkov-Merritt model
- Sersic profile
External links
- Elliptical Galaxies, SEDS Messier pages
- Elliptical Galaxies
References
- ↑ Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 36018182.
- ↑ Loveday, J. (February 1996). "The APM Bright Galaxy Catalogue.". Monthly Notices of the Royal Astronomical Society 278 (4): 1025–1048. http://articles.adsabs.harvard.edu/full/1996MNRAS.278.1025L. Retrieved 2007-09-15.
- ↑ Dressler, A. (March 1980). "Galaxy morphology in rich clusters - Implications for the formation and evolution of galaxies.". The Astrophysical Journal 236: 351–365. doi:10.1086/157753. http://articles.adsabs.harvard.edu/full/1980ApJ...236..351D. Retrieved 2007-09-15.
- ↑ Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 9780691025650. OCLC 39108765.
- ↑ Merritt, D. (February 1999). "Elliptical galaxy dynamics". The Astronomical Journal 756: 129–168. doi:10.1086/316307. http://adsabs.harvard.edu/abs/1999PASP..111..129M.
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Book:Galaxies
Category:Galaxies