Dwarf galaxy

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The Large Magellanic Cloud, a satellite galaxy of the Milky Way. Picture taken by Hubble.

A 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. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

Formation of dwarf galaxies

Dwarf galaxy DDO 68.[1]

Current theory states that most galaxies, including dwarf galaxies, form in association with dark matter, or from gas that contains metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases lacking metals. These galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo.[2]

Because of their small size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies, resulting in galaxy merger.[3]

Local dwarf galaxies

The Phoenix Dwarf Galaxy is a dwarf irregular galaxy, featuring younger stars in its inner regions and older ones at its outskirts.[4]

There are many dwarf galaxies in the Local Group; these small galaxies frequently orbit larger galaxies, such as the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy. A 2007 paper[5] has suggested that many dwarf galaxies were created by galactic tides during the early evolutions of the Milky Way and Andromeda. Tidal dwarf galaxies are produced when galaxies collide and their gravitational masses interact. Streams of galactic material are pulled away from the parent galaxies and the halos of dark matter that surround them.[6]

More than 20 known dwarf galaxies orbit the Milky Way, and recent observations[7] have also led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with a black hole at its centre, which was at some time absorbed by the Milky Way.

Common dwarf galaxy types

UGC 11411 is a galaxy known as an irregular blue compact dwarf (BCD) galaxy.[8]

Blue compact dwarf galaxies

Blue compact dwarf PGC 51017.[10]

In astronomy, a blue compact dwarf galaxy (BCD galaxy) is a small galaxy which contains large clusters of young, hot, massive stars. These stars, the brightest of which are blue, cause the galaxy itself to appear blue in colour.[11] Most BCD galaxies are also classified as dwarf irregular galaxies or as dwarf lenticular galaxies. Because they are composed of star clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become very violent when forming.

BCD galaxies cool in the process of forming new stars. The galaxies' stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies.

Nearby examples include NGC 1705, NGC 2915, NGC 3353 and UGCA 281.[12][13][14][15]

Ultra-compact dwarfs

Ultra-compact dwarf galaxies (UCD) are a recently discovered[16] class of very compact galaxies with very high stellar populations. They are thought to be on the order of 200 light years across, containing about 100 million stars.[17] It is theorised that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars by tidal interactions, travelling through the hearts of rich clusters.[18] UCDs have been found in the Virgo Cluster, Fornax Cluster, Abell 1689, and the Coma Cluster, amongst others.[19] An extreme UCD example is M60-UCD1, about 54 million light years away, which contains approximately 200 million solar masses within a 160 light year radius and its central region packs in stars about 25 times closer together than stars in Earth's region in the Milky Way.[20][21] M59-UCD3 is approximately the same size as M60-UCD1 with a half-light radius, rh, of approximately 20 parsecs but is 40% more luminous with an apparent relative magnitude of approximately −14.6. This makes M59-UCD3 the densest known galaxy.[22]

Partial list of dwarf galaxies

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See also

References

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  17. ↑ Anglo-Australian Observatory Astronomers discover dozens of mini-galaxies 0100 AEST Friday 2 April 2004.
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External links

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