Starburst Galaxies
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A starburst galaxy is one undergoing an exceptionally high rate of
star formation Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...
, as compared to the long-term average rate of star formation in the
galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
, or the star formation rate observed in most other galaxies. For example, the star formation rate of the
Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
galaxy is approximately 3 M/yr, while starburst galaxies can experience star formation rates of 100 M/yr or more. In a starburst galaxy, the rate of star formation is so large that the galaxy consumes all of its gas reservoir, from which the stars are forming, on a timescale much shorter than the age of the galaxy. As such, the starburst nature of a galaxy is a phase, and one that typically occupies a brief period of a galaxy's evolution. The majority of starburst galaxies are in the midst of a
merger Mergers and acquisitions (M&A) are business transactions in which the ownership of a company, business organization, or one of their operating units is transferred to or consolidated with another entity. They may happen through direct absorpt ...
or
close encounter In ufology, a close encounter is an event in which a person witnesses an unidentified flying object (UFO) at relatively close range, where the possibility of mis-identification is presumably greatly reduced. This terminology and the system of cla ...
with another galaxy. Starburst galaxies include M82, NGC 4038/NGC 4039 (the Antennae Galaxies), and
IC 10 IC 10 is an irregular galaxy in the constellation Cassiopeia. It was discovered by American astronomer Lewis Swift on 8 October 1887. In 1935 Nicholas Mayall became the first to suggest that the object is extragalactic. Edwin Hubble suspected i ...
.


Definition

Starburst galaxies are defined by these three interrelated factors: # The rate at which the galaxy is currently converting gas into stars (the star-formation rate, or SFR). # The available quantity of gas from which stars can be formed. # A comparison of the timescale on which star formation consumes the available gas with the age or rotation period of the galaxy. Commonly used definitions include: * Continued star-formation where the current SFR would exhaust the available gas reservoir in much less time than the age of the Universe (the Hubble Time). * Continued star-formation where the current SFR would exhaust the available gas reservoir in much less time than the dynamical timescale of the galaxy (perhaps one rotation period in a disk type galaxy). * The current SFR, normalized by the past-averaged SFR, is much greater than unity. This ratio is referred to as the "birthrate parameter".


Triggering mechanisms

Mergers and tidal interactions between gas-rich galaxies play a large role in driving starbursts. Galaxies in the midst of a starburst frequently show tidal tails, an indication of a close encounter with another galaxy, or are in the midst of a merger. Turbulence, along with variations of time and space, cause the dense gas within a galaxy to compress and rapidly increase star formation. The efficiency at which the galaxy forms also increases its SFR . These changes in the rate of star formation also led to variations with depletion time, and power a starburst with its own galactic mechanisms rather than merging with another galaxy. Interactions between galaxies that do not merge can trigger unstable rotation modes, such as the bar instability, which causes gas to be funneled towards the nucleus and ignites bursts of star formation near the galactic nucleus. It has been shown that there is a strong correlation between the lopsidedness of a galaxy and the youth of its stellar population, with more lopsided galaxies having younger central stellar populations. As lopsidedness can be caused by tidal interactions and mergers between galaxies, this result gives further evidence that mergers and tidal interactions can induce central star formation in a galaxy and drive a starburst.


Types

Classifying types of starburst galaxies is difficult since starburst galaxies do not represent a specific type in and of themselves. Starbursts can occur in disk galaxies, and
irregular galaxies An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance ...
often exhibit knots of starburst spread throughout the irregular galaxy. Nevertheless, astronomers typically classify starburst galaxies based on their most distinct observational characteristics. Some of the categorizations include: * Blue compact galaxies (BCGs). These galaxies are often low mass, low metallicity, dust-free objects. Because they are dust-free and contain a large number of hot, young stars, they are often blue in optical and ultraviolet colours. It was initially thought that BCGs were genuinely young galaxies in the process of forming their first generation of stars, thus explaining their low metal content. However, old
stellar population In 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations. In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926. Baade observed tha ...
s have been found in most BCGs, and it is thought that efficient mixing may explain the apparent lack of dust and metals. Most BCGs show signs of recent mergers and/or close interactions. Well-studied BCGs include IZw18 (the most metal poor galaxy known), ESO338-IG04 and Haro 11. ** Blue compact dwarf galaxies (BCD galaxies) are small compact galaxies. ** Green Pea galaxies (GPs) are small compact galaxies resembling primordial starbursts. They were found by citizen scientists taking part in the Galaxy Zoo project. ** Blueberry galaxies (BBs) are dwarf starbursts that are low redshift counterparts of GPs and likely analogs of high redshift galaxies. While BBs have a low SFR, probably because of their youth and small masses, their sSFR is high and comparable to GPs. * Luminous infrared galaxies (LIRGs). ** Ultra-luminous Infrared Galaxies (ULIRGs). These galaxies are generally extremely dusty objects. The ultraviolet radiation produced by the obscured star-formation is absorbed by the dust and reradiated in the infrared spectrum at wavelengths of around 100 micrometres. This explains the extreme red colours associated with ULIRGs. It is not known for sure that the UV radiation is produced purely by star-formation, and some astronomers believe ULIRGs to be powered (at least in part) by
active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars. Such e ...
(AGN). X-ray observations of many ULIRGs that penetrate the dust suggest that many starburst galaxies are double-cored systems, lending support to the hypothesis that ULIRGs are powered by star-formation triggered by major mergers. Well-studied ULIRGs include
Arp 220 Arp 220 is the result of a collision between two galaxies which are now in the process of merging. It is the 220th object in Halton Arp's '' Atlas of Peculiar Galaxies''. Features Arp 220 is the closest ultraluminous infrared galaxy (ULIRG) ...
. ** Hyperluminous Infrared galaxies (HLIRGs), sometimes called submillimeter galaxies. * Wolf–Rayet galaxies (WR galaxies), galaxies where a large portion of the bright stars are
Wolf–Rayet star Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectroscopy, spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very ...
s. The Wolf–Rayet phase is a relatively short-lived phase in the life of massive stars, typically 10% of the total life-time of these stars and as such any galaxy is likely to contain few of these. However, because the stars are both luminous and have distinctive spectral features, it is possible to identify these stars in the spectra of entire galaxies and doing so allows good constraints to be placed on the properties of the starbursts in these galaxies.


Ingredients

Firstly, a starburst galaxy must have a large supply of gas available to form stars. The burst itself may be triggered by a close encounter with another galaxy (such as M81/M82), a collision with another galaxy (such as the Antennae), or by another process that forces material into the centre of the galaxy (such as a stellar bar). The inside of the starburst is quite an extreme environment. The large amounts of gas mean that massive stars are formed. Young, hot stars ionize the gas (mainly
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
) around them, creating
H II region An H II region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light year ...
s. Groups of hot stars are known as OB associations. These stars burn bright and fast, and are quite likely to explode at the end of their lives as
supernovae A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original ob ...
. After the supernova explosion, the ejected material expands and becomes a
supernova remnant A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...
. These remnants interact with the surrounding environment within the starburst (the
interstellar medium The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...
) and can be the site of naturally occurring
masers A maser is a device that produces coherent electromagnetic waves ( microwaves), through amplification by stimulated emission. The term is an acronym for microwave amplification by stimulated emission of radiation. Nikolay Basov, Alexander Pro ...
. Studying nearby starburst galaxies can help us determine the history of galaxy formation and evolution. Large numbers of the most distant galaxies seen, for example, in the
Hubble Deep Field The Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. It covers an area about 2.6 arcminutes on a side, about one 24-millionth of the ...
are known to be starbursts, but they are too far away to be studied in any detail. Observing nearby examples and exploring their characteristics can give us an idea of what was happening in the early universe as the light we see from these distant galaxies left them when the universe was much younger (see
redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...
). However, starburst galaxies seem to be quite rare in our local universe, and are more common further away – indicating that there were more of them billions of years ago. All galaxies were closer together then, and therefore more likely to be influenced by each other's gravity. More frequent encounters produced more starbursts as galactic forms evolved with the expanding universe.


Examples

M82 is the archetypal starburst galaxy. Its high level of star formation is due to a close encounter with the nearby spiral M81. Maps of the regions made with radio telescopes show large streams of neutral hydrogen connecting the two galaxies, also as a result of the encounter. Radio images of the central regions of M82 also show a large number of young supernova remnants, left behind when the more massive stars created in the starburst came to the end of their lives. The Antennae is another starburst system, detailed by a Hubble picture, released in 1997.


List of starburst galaxies


Gallery

File:Potw1629a.jpg, In NGC 3125 unusually high numbers of new stars forming occurs. File:Starburst galaxy MCG+07-33-027.jpg, Starburst galaxy MCG+07-33-027. File:A swirl of star formation.jpg, J125013.50+073441.5 taken by Hubble as part of a study named LARS (Lyman Alpha Reference Sample) File:NGC 1569.jpg, Starburst activity in the central region of nearby
dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...
NGC 1569 (Arp 210). Taken by
Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
. File:Ssc2008-12a small.jpg, As viewed from our position 12.2
billion Billion is a word for a large number, and it has two distinct definitions: * 1,000,000,000, i.e. one thousand million, or (ten to the ninth power), as defined on the short scale. This is now the most common sense of the word in all varieties of ...
light years A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astro ...
away, the Baby Boom Galaxy is seen to be creating 4,000 stars per year. Credit:
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
. File:Starburst in NGC 4449 (captured by the Hubble Space Telescope).jpg, The galaxy NGC 4449 is currently a ''global'' starburst, with star formation activity widespread throughout the galaxy. File:HXMM01.jpg, Explosive star formation in the currently merging galaxy HXMM01 11 billion
light years A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astro ...
away. Captured by
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
. File:ESO Centaurus A LABOCA.jpg, An image of the galaxy
Centaurus A Centaurus A (also known as NGC 5128 or Caldwell 77) is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable ...
made by combining images from the
MPG/ESO telescope The MPG/ESO telescope is a 2.2-metre f/8.0 (17.6-metre) ground-based telescope at the European Southern Observatory (ESO) in La Silla, Chile. It was built by Zeiss and has been operating since 1984. It was on indefinite loan to the European Sout ...
, and the
Chandra X-ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...
. It is the only known case of an " Elliptical Starburst" galaxy.


See also

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Notes


References

* * * {{DEFAULTSORT:Starburst Galaxy Articles containing video clips