Galaxy undergoing an exceptionally high rate of star formation
TheAntennae Galaxies are an example of a starburst galaxy occurring from the collision of NGC 4038/NGC 4039. Credit:NASA/ESA.
Astarburst galaxy is one undergoing an exceptionally high rate ofstar formation, as compared to the long-term average rate of star formation in thegalaxy, or the star formation rate observed in most other galaxies.
For example, the star formation rate of theMilky Way galaxy is approximately 3M☉/yr, while starburst galaxies can experience star formation rates of 100 M☉/yr or more.[1] 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, thestarburst nature of a galaxy is a phase, and one that typically occupies a brief period of agalaxy's evolution. The majority of starburst galaxies are in the midst of amerger orclose encounter with another galaxy. Starburst galaxies includeM82,NGC 4038/NGC 4039 (the Antennae Galaxies), andIC 10.
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".
In today's Universe, tidal interactions between gas-rich galaxies play a large role in driving starbursts. Galaxies in the midst of a starburst frequently showtidal tails, an indication of a close encounter with another galaxy, or are in the midst of a merger. Tidal gravitation forces or direct collisions between gas clouds compress gas within a galaxy, and the compressed gas rapidly forms stars. The overall efficiency of converting gas to stars also increases. 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.[3] 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.[4] 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. At earlier cosmic epochs, when galaxies generally had higher gas content, mergers may have been less important in triggering starbursts, but the evidence is unclear.
Artist's impression of a galaxy undergoing a starburst[5]
Classifying types of starburst galaxies is difficult because starburst galaxies do not represent a specific type in and of themselves. Starbursts can occur indisk galaxies, andirregular galaxies 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, oldstellar populations 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 includeIZw18 (the most metal poor galaxy known),ESO338-IG04, andHaro 11.
Green Pea galaxies (GPs) are small compact galaxies resembling primordial starbursts. They were found by citizen scientists taking part in theGalaxy 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). These galaxies are by definition extremely dusty objects with luminosities greater than 1010L☉. The ultraviolet radiation produced by the luminous young stars is absorbed by the dust and re-radiated in theinfrared at wavelengths of around 100 micrometers. Dust absorbs blue and ultraviolet light better than red light, and this explains the extreme red colors associated with ULIRGs. Not all of the original UV radiation is necessarily produced purely by star formation, and some LIRGs are likely powered in part byactive galactic nuclei (AGN).
Ultra-luminous Infrared Galaxies (ULIRGs). These are more extreme versions of LIRGs with luminosities greater than 1011L☉.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. One well-studied ULIRG isArp 220.
Submillimeter galaxies (SMGs) are ULIRGs or HyLIRGs atredshift larger than 2. At these redshifts, the 100 micrometer infrared emission is observed at sub-millimeter wavelengths, making this wavelength range favorable for detecting distant ULIRGs.
SBS 1415+437 is a WR galaxy located about 45 million light-years from Earth.[6]
Wolf–Rayet galaxies (WR galaxies), galaxies where a large portion of the bright stars areWolf–Rayet stars. 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,[7] 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.
Messier 82 is the prototype nearby starburst galaxy about 12 million light-years away in the constellationUrsa Major.
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 (mainlyhydrogen) around them, creatingH II regions. Groups of hot stars are known asOB associations. These stars burn bright and fast, and are quite likely to explode at the end of their lives assupernovae.
After the supernova explosion, the ejected material expands and becomes asupernova remnant. These remnants interact with the surrounding environment within the starburst (theinterstellar medium) and can be the site of naturally occurringmasers.
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 theHubble Deep Field 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 (seeredshift). 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.
Artist's impression of gas fueling distant starburst galaxies[8]
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.[9] 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.[10]
This is adynamic list and may never be able to satisfy particular standards for completeness. You can help byediting the page to add missing items, with references toreliable sources.
This video zooms into distant galaxies undergoing a starburst in a region of sky known as the ExtendedChandra Deep Field South, in the constellation ofFornax (The Furnace).
^Sakamoto, Kazushi; Ho, Paul T. P.; Iono, Daisuke; Keto, Eric R.; Mao, Rui-Qing; Matsushita, Satoki; Peck, Alison B.; Wiedner, Martina C.; Wilner, David J.; Zhao, Jun-Hui (10 January 2006). "Molecular Superbubbles in the Starburst Galaxy NGC 253".The Astrophysical Journal.636 (2):685–697.arXiv:astro-ph/0509430.Bibcode:2006ApJ...636..685S.doi:10.1086/498075.S2CID14273657.
Weedman, D. W.; Feldman, F. R.; Balzano, V. A.; Ramsey, L. W.; Sramek, R. A.; Wuu, C. -C. (1981). "NGC 7714 – the prototype star-burst galactic nucleus".The Astrophysical Journal.248: 105.Bibcode:1981ApJ...248..105W.doi:10.1086/159133.
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![In NGC 3125 unusually high numbers of new stars forming occurs.[15]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPotw1629a.jpg)
![Starburst galaxy MCG+07-33-027.[16]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aStarburst_galaxy_MCG%252B07-33-027.jpg)
![J125013.50+073441.5 taken by Hubble as part of a study named LARS (Lyman Alpha Reference Sample)[17]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aA_swirl_of_star_formation.jpg)
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