An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his

Hubble sequence The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a t ...

and 1936 work ''The Realm of the Nebulae'',Alt URL
pp. 124–151) along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with

lenticular galaxies A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. ...

(S0) with their large-scale disks, and ES galaxiesLiller, M.H. (1966)
The Distribution of Intensity in Elliptical Galaxies of the Virgo Cluster. II
/ref> with their intermediate scale disks, a subset of the "early-type" galaxy population. Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity, and they tend to be surrounded by large numbers of

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

s. Elliptical galaxies are believed to make up approximately 10–15% of galaxies in the

Virgo Supercluster The Virgo Supercluster (Virgo SC) or the Local Supercluster (LSC or LS) is a mass concentration of galaxies containing the Virgo Cluster and Local Group, which itself contains the Milky Way and Andromeda galaxies, as well as others. At least ...

, and they are not the dominant type of galaxy in the universe overall. They are preferentially found close to the centers of

galaxy cluster A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-lar ...

s. Elliptical galaxies range in size from dwarf ellipticals with tens of millions of stars, to

supergiants Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars ...

of over one hundred trillion stars that dominate their galaxy clusters. Originally, Edwin Hubble hypothesized that elliptical galaxies evolved into

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''John, D. (2006). ''Astronomy: The definitive guide to the universe''. Bath, UK: Parragon Publishing., p. 224-225 although the accretion of gas and smaller galaxies may build a disk around a pre-existing ellipsoidal structure. Stars found inside of elliptical galaxies are on average much older than stars found in spiral galaxies.

Examples

* 3C 244.1 * M49 * M59 * M60 (NGC 4649) * M87 (NGC 4486), whose supermassive black hole is the first black hole to be imaged by the

Event Horizon Telescope The Event Horizon Telescope (EHT) is a large Astronomical interferometer, telescope array consisting of a global network of radio telescopes. The EHT project combines data from several very-long-baseline interferometry (VLBI) stations around Ear ...

. * M89 * M105 (NGC 3379) *

IC 1101 IC 1101 is a class S0 supergiant ( cD) lenticular galaxy at the center of the Abell 2029 galaxy cluster. It has an isophotal diameter at about . It possesses a diffuse core which is the largest known core of any galaxy to date, and also hosts a ...

, one of the largest galaxies in the

observable universe The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these ob ...

. *

Hercules A Hercules A is a bright astronomical radio source within the vicinity of the constellation Hercules corresponding to the galaxy 3C 348. Observation During a survey of bright radio sources in the mid-20th century, astronomers found a very brig ...

, supergiant elliptical galaxy. *

Maffei 1 Maffei 1 is a massive elliptical galaxy in the constellation Cassiopeia. Once believed to be a member of the Local Group of galaxies, it is now known to belong to a separate group, the IC 342/Maffei Group. It was named after Paolo Maffei, who ...

, the closest giant elliptical galaxy. * CGCG 049-033, known for having the longest galactic jet discovered. * Centaurus A (NGC 5128), an elliptical/lenticular

radio galaxy A radio galaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from its active galactic nucleus. They have luminosities up to 1039 W at radio wav ...

with peculiar morphology and unusual dust lanes * NeVe 1, the source of the Ophiuchus Supercluster eruption, the most powerful astronomical event known.

General characteristics

Elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. They are spherical or ovoid masses of stars, starved of star-making gases. Furthermore, there is very little

interstellar matter In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar ...

(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 colors. Large elliptical galaxies typically have an extensive system of

s. They generally have two distinct populations of globular clusters: one that is redder and metal-rich, and another that is bluer and metal-poor. The dynamical properties of elliptical galaxies and the bulges of disk galaxies are similar, suggesting that they may be 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, and a range of scaling relations between the elliptical galaxies' structural parameters unify the population. Every massive elliptical galaxy contains a

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions of times the mass of the Sun (). Black holes are a class of astronomical ob ...

at its center. Observations of 46 elliptical galaxies, 20 classical bulges, and 22 pseudobulges show that each contain a black hole at the center. The mass of the black hole is tightly correlated with the mass of the galaxy, evidenced through correlations such as the

M–sigma relation The M–sigma (or ''M''–''σ'') relation is an empirical correlation between the stellar velocity dispersion ''σ'' of a galaxy bulge and the mass M of the supermassive black hole at its center. The ''M''–''σ'' relation was first presented ...

which relates the

velocity dispersion In astronomy, the velocity dispersion (''σ'') is the statistical dispersion of velocities about the mean velocity for a group of astronomical objects, such as an open cluster, globular cluster, galaxy, galaxy cluster, or supercluster. By measurin ...

of the surrounding stars to the mass of the black hole at the center. Elliptical galaxies are preferentially found in

galaxy clusters A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-l ...

and in compact groups of galaxies. Unlike flat

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae'' The largest galaxies are supergiant ellipticals, or type-cD galaxies. Elliptical galaxies vary greatly in both size and mass with diameters ranging from 3,000 light years to more than 700,000 light years, and masses from 10⁵ to nearly 10¹³ solar masses. This range is much broader for this galaxy type than for any other. The smallest, the

dwarf elliptical galaxies Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are smaller than ordinary elliptical galaxies. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies. Examples "Dwarf elliptical" galaxie ...

, may be no larger than a typical

, but contain a considerable amount of

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

not present in clusters. Most of these small galaxies may not be related to other ellipticals. MACSJ1423

The

Hubble classification The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a t ...

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: :

10 \times \left(1 - \frac\right)

Thus for a spherical galaxy with ''a'' equal to ''b'', the number is 0, and the Hubble type is E0. While the limit in the literature is about E7, it has been known since 1966 that the E4 to E7 galaxies are misclassified lenticular galaxies with disks inclined at different angles to our line of sight. This has been confirmed through spectral observations revealing the rotation of their stellar disks. 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. It is sometimes said that there are two physical types of ellipticals: the giant ellipticals with slightly "boxy"-shaped isophotes, whose shapes result from random motion which is greater in some directions than in others (anisotropic random motion); and the "disky" normal and dwarf ellipticals, which contain disks. This is, however, an abuse of the nomenclature, as there are two types of early-type galaxy, those with disks and those without. Given the existence of ES galaxies with intermediate-scale disks, it is reasonable to expect that there is a continuity from E to ES, and onto the S0 galaxies with their large-scale stellar disks that dominate the light at large radii. 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's classification. Beyond ''gE'' giant ellipticals, lies D-galaxies and cD-galaxies. These are similar to their smaller brethren, but more diffuse, with large haloes that may as much belong to the galaxy cluster within which they reside than the centrally-located giant galaxy. A galactic mega-merger

Star formation

In recent years, evidence has shown that a reasonable proportion (~25%) of early-type (E, ES and S0) galaxies have residual gas reservoirs and low level star-formation. Herschel Space Observatory researchers have speculated that the central black holes in elliptical galaxies keep the gas from cooling enough for star formation."Red And Dead Galaxies Have Beating Black Hole 'Hearts', Preventing Star Formation."
/ref>

References

External links

Elliptical Galaxies
SEDS Messier pages

{{Galaxy Galaxy morphological types

Examples

General characteristics

Star formation

See also

References

Further reading

External links