Dwarf elliptical galaxy
Encyclopedia
Dwarf elliptical galaxies, or dEs, are elliptical galaxies
that are much smaller than others. They are classified as dE, and are quite common in galaxy groups and clusters, and are usually companions to other galaxies.
, a satellite of the Andromeda galaxy
. In 1944 Walter Baade
confirmed dwarf ellipticals NGC147 and NGC185 as members of the Local Group
by resolving them into individual stars. Resolving stars in NGC147 and NGC185 was only possible because these dEs are very nearby galaxies. In the 1950s, dEs were also discovered in the nearby Fornax and Virgo clusters.
s within the range -18 mag < M < -14 mag, fainter than giant elliptical galaxies. While the surface brightness profiles of giant elliptical galaxies
are well described by de Vaucouleur's law, dEs have exponentially declining surface brightness profiles. However, both types can be well fit by the same more general law, Sersic's law, and there is a continuity of Sersic index as a function of luminosity, suggesting that dwarf and giant elliptical galaxies belong to a single sequence. Still fainter elliptical-like galaxies, called dwarf spheroidal galaxies, appear to be genuinely distinct.
, small objects (consisting of dark matter and gas) are the first to form. Because of their mutual gravitational attraction, some of these will coalesce and merge, forming more massive objects. Further mergers lead to ever more massive objects.
The process of coalescence is thought to lead to the present-day galaxies, and has been called "hierarchical merging". If this hypothesis is correct, dwarf galaxies may be the building blocks of today's giant galaxies.
An alternative suggestion is that dEs could be the remnants of low-mass spiral galaxies that obtained a rounder shape through the action of repeated gravitational interactions with giant galaxies within a cluster. This process of changing a galaxy's morphology by interactions has been called "galaxy harassment". Evidence for this latter hypothesis has been found in the form taken by stellar disks and spiral arms of spiral galaxies
. Under this alternative hypothesis the disks and arms are modified version of the original stellar disk of the transformed spiral galaxy, and similarly, small remnants of disks and arms are embedded within "harassed" dEs.
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...
that are much smaller than others. They are classified as dE, and are quite common in galaxy groups and clusters, and are usually companions to other galaxies.
Examples
One of the most nearby Dwarf ellipticals (dEs) is M32Messier 32
Messier 32 is a dwarf elliptical galaxy about 2.65 million light-years away in the constellation Andromeda. M32 is a satellite galaxy of the famous Andromeda Galaxy and was discovered by Le Gentil in 1749. M32 measures only 6.5 ± 0.2 kly in diameter at the widest point...
, a satellite of the Andromeda galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
. In 1944 Walter Baade
Walter Baade
Wilhelm Heinrich Walter Baade was a German astronomer who worked in the USA from 1931 to 1959.-Biography:He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the...
confirmed dwarf ellipticals NGC147 and NGC185 as members of the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
by resolving them into individual stars. Resolving stars in NGC147 and NGC185 was only possible because these dEs are very nearby galaxies. In the 1950s, dEs were also discovered in the nearby Fornax and Virgo clusters.
Comparison with giant ellipticals
Dwarf elliptical galaxies have blue absolute magnitudeAbsolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
s within the range -18 mag < M < -14 mag, fainter than giant elliptical galaxies. While the surface brightness profiles of giant elliptical galaxies
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...
are well described by de Vaucouleur's law, dEs have exponentially declining surface brightness profiles. However, both types can be well fit by the same more general law, Sersic's law, and there is a continuity of Sersic index as a function of luminosity, suggesting that dwarf and giant elliptical galaxies belong to a single sequence. Still fainter elliptical-like galaxies, called dwarf spheroidal galaxies, appear to be genuinely distinct.
Two hypotheses for origins
Dwarf ellipticals may be primordial objects. Within the currently favoured cosmological Lambda-CDM modelLambda-CDM model
ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...
, small objects (consisting of dark matter and gas) are the first to form. Because of their mutual gravitational attraction, some of these will coalesce and merge, forming more massive objects. Further mergers lead to ever more massive objects.
The process of coalescence is thought to lead to the present-day galaxies, and has been called "hierarchical merging". If this hypothesis is correct, dwarf galaxies may be the building blocks of today's giant galaxies.
An alternative suggestion is that dEs could be the remnants of low-mass spiral galaxies that obtained a rounder shape through the action of repeated gravitational interactions with giant galaxies within a cluster. This process of changing a galaxy's morphology by interactions has been called "galaxy harassment". Evidence for this latter hypothesis has been found in the form taken by stellar disks and spiral arms of spiral galaxies
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
. Under this alternative hypothesis the disks and arms are modified version of the original stellar disk of the transformed spiral galaxy, and similarly, small remnants of disks and arms are embedded within "harassed" dEs.
See also
- Dwarf galaxyDwarf galaxyA dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...
- Dwarf spheroidal galaxyDwarf spheroidal galaxyDwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
- Elliptical galaxyElliptical galaxyAn 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...
- Galaxy morphological classification
- Irregular galaxyIrregular galaxyAn irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...