A starburst galaxy is a galaxy in the process of an intense burst of star formation, often as a result of a collision or close encounter between two galaxies.
Normal galaxies also form stars, but at a much lower rate. The star formation is so intense in a starburst that, if it 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. This is why they are known as "starbursts". Well-known starburst galaxies include M82 and IC 10.
A subtype of starburst galaxy is the Wolf-Rayet galaxy, where a large portion of stars of the starburst are
Definitions of starburst
Several definitions of the term starburst galaxy exist and there isn't really a strict definition on which all astronomers
agree. However, they would generally agree that the definition must in some way be related to (i) the rate at which the galaxy
is currently converting gas into stars -- the star-formation rate (SFR) (ii) the available quantity of gas from which
stars can be formed (iii) perhaps some additional timescales. 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
Starburst triggering mechanisms
Essentially to ignite a starburst, it is necessary to concentrate a lot 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 circumnuclear starbursts.
Types of starburst
Classifying the starburst category itself isn't easy as starburst galaxies don't represent a specific type in themselves:
starbursts can occur in disk galaxies and irregulars often exhibit global starburst of starbursting knots. However, several
different subtypes of starburst are currently in play:
- 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 merger and/or close interaction. 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 regime 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
M82 is the prototypical 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.
The Antennae is another well-known starburst system, made famous by a stunning Hubble picture, released in 1997.
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 centre 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 ionise the gas (mainly hydrogen) around them creating HII regions. Groups of very hot stars are known as OB associations These stars burn very bright and
very fast (live fast, die young!), 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).