Dwarf galaxy

Dwarf galaxies are extragalactic stellar systems that lie between the smaller globular clusters and the larger "normal" galaxies. In general, it is now believed that there are no fundamental differences between "normal" galaxies and dwarf galaxies as systems and between them make their properties a continuous transition.

While in the dwarf galaxies, however, with decreasing absolute brightness decreases their surface brightness (ie luminosity density), takes on the big elliptical galaxies with increase in the absolute brightness from its surface brightness. Criteria such as the absolute magnitude, the surface brightness, half-light radius, diameter, mass, or the number of stars to serve the objectives and requirements of the particular investigation.

3.1 Ultra- faint dwarf galaxies
3.2 Low surface dwarf spheroidal galaxy
3.3 Blue compact dwarf galaxies
3.4 Ultra- compact dwarf galaxies

Occurrence

Dwarf galaxies are found as satellites of large galaxies, as in the case of the Magellanic Clouds and the Milky Way. They are both in the walls of the gigantic empty spaces of the universe - especially often found in galaxy groups and clusters of galaxies, and - - there dominate dwarf elliptical galaxies there usually irregular dwarf galaxies. Dwarf galaxies are significantly, by approximately a factor of 10, more frequently than the large galaxies. In larger distances and due to their low surface brightness, they are more difficult to observe.

Known as relatively bright examples are the two companions M 32 and M 110 of the Andromeda nebula or the Sagittarius dwarf galaxy, which are among the 24 known dwarf galaxies orbiting the Milky Way. The Local Group has a total of at least 67 known dwarf galaxies to their members.

It is expected that more dwarf galaxies are also found in the Local Group. For example, in the current situation was the nearest dwarf galaxy, the Canis Major dwarf galaxy, discovered only in 2003. For the dwarf galaxies of the Milky Way are very close, it is often very difficult to distinguish the galaxy from the foreground star of the Milky Way. Another example of a very close and hard to observed dwarf galaxy is the Draco dwarf galaxy.

Properties

Morphology

Morphologically, one divides dwarf galaxies according to their shape in

Spheroidal ( dSph ) and
Irregular ( dIrr ) dwarf galaxies,
And the relatively rare dwarf spiral galaxies ( dS )

Elliptic and spheroidal dwarf galaxies are summarized as earlier morphological type, the irregular and spiral dwarf galaxies than later morphological type. Besides, the early types are currently showing no star formation, while the appearance of late types is often dominated by massive, young stars. However, during its development, it was also used in the dwarf galaxies early morphological type in part to several phases of star formation.

Composition

The gas and dust content in dwarf galaxies early morphological type is lower than that of the late type. The proportion of dark matter in dwarf galaxies is often 10 to 100 times higher than in normal galaxies. For example, the spheroidal dwarf galaxy Segue 1 in the Local Group about 1000 times as much dark as visible matter, the normal galaxies in only about 10 times as much.

The star number of a dwarf galaxy is only between some 100,000 and several hundred million suns, while our spiral Milky Way galaxy about 300 billion suns and is one of the large elliptical galaxies may include even trillions of suns.

Formation

Like most galaxies, so arise, according to new theoretical models, the dwarf galaxies where the gas collapses due to dark matter or by increased metallicity and thereby sufficiently high density for the star formation occurs (→ Jeans criterion). Recent discoveries, however, show how the so-called Leo ring also from metal-poor primordial gas clouds near two large galaxies in the constellation Leo new dwarf galaxies triggered by tidal forces density fluctuations arise.

Dwarf galaxies are gravitationally bound only weakly due to their small number of stars and lose in a short time the entire interstellar matter to form new generations of stars. Due to the low internal gravitational binding the dwarf galaxies are destroyed by close encounters of larger galaxies. A resolution dwarf galaxy appears as a stream of stars, where the stars still show a similar proper motion, but are already distributed over a large region of space. An example of this is the Sagittarius dwarf galaxy with the Sagittarius stream. In the course of some 100 million years, the star power dissolves and the stars shall become the galactic halo. The Galactic Halo with its old stars and some retrograde orbits should mainly consist of the remains of dwarf galaxies.

Special shapes

Ultra faint dwarf galaxies

The faintest known dwarf galaxies, the ultra faint dwarf galaxies ( UFD, dt " ultra- faint dwarf galaxies " ), show a surface brightness of 28 likes per square arcsecond or less. Their luminosity is the thousand-fold solar luminosities below those of globular clusters. During the half-light radius of globular clusters is usually less than 50 parsecs, the radius of ultra- faint dwarf galaxies reach up to 1,000 parsecs. Is correspondingly low, the density of stars in these dwarf galaxies and their velocity dispersion is less than 4 km / s Their shape is often irregular due to gravitational interactions with the host galaxy, which they orbit. The low density is a result of a broken stellar evolution, since the massive stars of the first generation already can accelerate to escape velocity in the explosion as a supernova or by the fast stellar wind interstellar matter. As a result, the gravitational interaction leads to the disk of the Milky Way to a widening in the direction of movement. This is called a stellar stream that mixes in the halo of the galaxy over the course of billions of years.

The stellar population in the UFD is usually older than 10 billion years. The only known exception is Leo T, in which it probably still comes to star formation.

Low surface dwarf spheroidal galaxy

Low surface spheroidal dwarf galaxies are extremely faint dwarf lenticular galaxies with a luminosity of a few hundred thousand solar luminosities. This value is lower than that of globular clusters and some of the open star clusters. The diameter of the core is 300 to 1000 dSph Parsecs, said surrounded halo can be detected in a three -to ten- fold greater in diameter. From dSph only electromagnetic radiation can the stars be detected and there are no signs of interstellar matter or dust in these Stellarsystemen. The typical age of dSph in the Local Group is 1 billion years.

Blue compact dwarf galaxies

Blue compact dwarf galaxies ( BCDs, dt "blue compact dwarf galaxies " ) are small, compact galaxies, called the great young star clusters containing massive stars. The brightest of these stars are blue according to their mass and let the entire galaxy are blue. Most BCD galaxies are classified as dwarf irregular galaxies, since they are composed of several of these star-forming regions and therefore do not form a regular shape.

BCD galaxies cool while forming new stars are continuously under heavy consumption of their interstellar gas. With the progressive development of these galaxies then changes the form of this type galaxies.

To the nearest examples of this type include the galaxies NGC 1705 and NGC 2915th

Ultra compact dwarf galaxies

Ultra compact dwarf galaxies ( UCDs, dt " ultra dense dwarf galaxies " ) are a recently discovered in 2003, a special form of dwarf galaxies with extremely high, central stellar density. Up to 100 million stars are located within their small diameter, which is below the order of 200 light years. According to current theoretical models of these galaxies were their outer regions ( both gas, dust and stars) by tidal action during their passages inside their dense clusters of galaxies, where they are located, robbed. Accordingly, some of them were in the Virgo cluster of galaxies, the Fornax cluster of galaxies, identified in Abell 1689 and the Coma cluster of galaxies.

Globular clusters
Structure of the cosmos
Dark Matter
Hubble sequence
List of galaxies of the Local Group
List of satellite galaxies of the Milky Way
List of satellite galaxies of Andromeda

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