A starburst galaxy is one undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy, or the star formation rate observed in most other galaxies.
For example, the star formation rate of the Milky Way galaxy is approximately 3 <var>M</var><sub>âÂÂ</sub>/yr, while starburst galaxies can experience star formation rates of 100 M<sub>âÂÂ</sub>/yr or more. While many main-sequence galaxies use up their molecular gas reservoir in 1-2 Gyr, the high rate of star formation in starbursts can deplete all of their star-forming material in scales of 0.1 Gyr - 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 or close encounter with another galaxy. Starburst galaxies include M82, NGC 4038/NGC 4039 (the Antennae Galaxies), and IC 10.
Starburst galaxies are defined by these three interrelated factors:
Commonly used definitions include:
The specific amount of contribution of starbursts to star formation is inconsistent across studies (partially due to inconsistencies in definitions of starbursts) but generally insignificant across all epochs - though some sources indicate a very high contribution at cosmic noon. In the local universe, starburst galaxies make up ~1% of all star-forming galaxies, and ~3-6% of all star formation; though identification of starbursts compared to AGNs due to both of their high luminosities can lead to some error. Generally, the fraction of starbursts increases with redshift, and increases for galaxies with lower initial mass functions. While some studies indicate a large starburst population at z ~ 5 (15% for galaxies with mass > ), this may be an overestimation.
Starbursts are triggered by processes that cause density surges in galaxies - usually mergers, gravitational disruptions, and stellar bar gas accumulation. A large portion of starbursts are caused by same or different mass mergers. 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. Tidal gravitation forces or direct collisions between gas clouds can cause dust and gas to lose angular momentum and fall inwards, 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. 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. 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. Additionally, the gas flows in the bars of spiral galaxies cause dust to accumulate towards the center of the galaxy, which can trigger starbursts in the centers of galaxies. This effect is most notable in galaxies with low redshifts.
Classifying types of starburst galaxies is difficult because starburst galaxies do not represent a specific type in and of themselves. Starbursts can occur in disk galaxies, and irregular 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:
First, 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 center 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) around them, creating H II regions. 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.
After the supernova explosion, the ejected material expands and becomes a supernova remnant. These remnants interact with the surrounding environment within the starburst (the interstellar medium) and can be the site of naturally occurring masers.
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 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).
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.
Starburst galaxies, as with all star-forming galaxies, can only form stars so long as they have fuel enough to do (see ingredients above). Starburst galaxies, due to the vigor of their star formation, can deplete their fuel in approximately an order of magnitude faster than regular star forming galaxies. Galaxies that have exhausted their star formation fuel and no longer show signs of star formation are known as quiescent or quenched galaxies; starburst galaxies that are quiescent/quenched are known as Post Starburst galaxies (PSB's). These PSB's are known to come about in part due to intense outflows of gas and dust caused by the resultant stellar winds of star formation. Recent survey's using the Atacama Large Millimeter/submillimeter Array (ALMA) show that intense magnetic fields in the starbursts also contribute to the strength of wind-outflow and the inevitable transition to a PSB.
PSB's are generally morphologically similar to other quiescent galaxies. Some findings suggest that PSB's are more spherical in nature than their quiescent counterparts and at moderate high star-forming redshifts (z~1.2) more compact. It is thought that these compact PSB's are the remnants of nuclear starbursts, or starbursts occurring as a result of gas losing angular momentum and cascading into the center of a galaxy at rates much faster than typical star-bursting galaxies.