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Starburst galaxy

The Antennae Galaxies are an example of a very high starburst galaxy occurring from the collision of NGC 4038/NGC 4039. Credit: NASA/ESA

A starburst galaxy is a galaxy in the process of an exceptionally high rate of star formation, compared to the usual star formation rate seen in most galaxies. Galaxies are often observed to have a burst of star formation after a collision or close encounter between two galaxies. The rate of star formation is so great for a galaxy undergoing a starburst that, if the rate was sustained, the gas reservoirs from which stars are formed would be used up on timescales much shorter than the dynamical lifetime of the galaxy. For this reason, it is presumed that starbursts are temporary. Well-known starburst galaxies include M82, NGC 4038/NGC 4039 (the Antennae Galaxies), and IC 10.

Starburst definitions

Several definitions of the term starburst galaxy exist and there isn't really a strict definition on which all astronomers agree. However, many generally agree that the definition must in some way be related to these three factors:

1. the rate at which the galaxy is currently converting gas into stars (the star-formation rate, or SFR)

2. the available quantity of gas from which stars can be formed

3. comparison of the timescale of star formation with the age or rotation period of the galaxy.

Commonly used definitions include:

* Continued star-formation with the current SFR would exhaust the available gas reservoir in much less than the age of the Universe (the Hubble Time). This is sometimes referred to as a "true" starburst.

* Continued star-formation with the current SFR would exhaust the available gas reservoir in much less than the dynamical timescale of the galaxy (perhaps one rotation period in a disk type galaxy).

* The current SFR, normalised by the past-averaged SFR is much greater than unity. This ratio is referred to as the birthrate parameter.

Starburst triggering mechanisms

Essentially to ignite a starburst, it is necessary to concentrate a large amount of cool molecular gas in a small volume. Such concentrations and perturbations are strongly suspected to cause global starburst phenomena in major galaxy mergers, although the exact mechanisms are not fully understood.

Observational surveys have long since shown that there is often a burst of disk star-formation in merging and interacting pairs of galaxies. It is also currently believed that nearby interactions between galaxies that don't actually merge can trigger unstable rotation modes, such as the bar instability, that cause gas to be funneled towards the nucleus, igniting bursts of star formation near the galactic nucleus.

Types of starburst

Classifying the starburst category itself isn't easy since starburst galaxies don't represent a specific type in themselves. Starbursts can occur in disk galaxies, and irregular galaxies often exhibit knots of starburst, often spread throughout the irregular galaxy. However, several different subtypes of starburst are currently under discussion among galactic astronomers:

* 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 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 include IZw18 (the most metal poor galaxy known), ESO338-IG04 and Haro11.

* 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 (AGN). X-ray observations of many ULIRGs that penetrate the dust suggest that many starburst 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.

* Wolf-Rayet galaxies, where a large portion of the bright stars are Wolf-Rayet stars.

The ingredients of a starburst

Firstly, a starburst 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 which forces material into the center of the galaxy (such as a stellar bar).

Inside the starburst is quite an extreme environment. The large amounts of gas mean that very massive stars are formed. Young, hot stars ionize the gas (mainly hydrogen) around them creating H II regions. Groups of very hot stars are known as OB associations. These stars burn very bright and very 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 very 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). Unfortunately 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.

Well-known starbursts

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 well-known starburst system, made famous by a stunning Hubble picture, released in 1997.