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 M☉/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, 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:
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.[2] 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.[3] 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.
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 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:
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) 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). 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.
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.[5] 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.[6]
| Galaxy | Type | Notes | Refs |
|---|---|---|---|
| M82 | I0 | Archetype starburst galaxy | |
| Antennae Galaxies | SB(s)m pec / SA(s)m pec | Two colliding galaxies | |
| IC 10 | dIrr | Mild starburst galaxy; nearest starburst galaxy and the only one in the Local Group. | |
| HXMM01 | Extreme starburst merging galaxies | ||
| HFLS3 | Unusually large intense starburst galaxy | ||
| NGC 1569 | IBm | Dwarf galaxy undergoing a galaxy-wide starburst | |
| NGC 2146 | SB(s)ab pec | ||
| NGC 1705 | SA0− pec | ||
| NGC 1614 | SB(s)c pec | Merging with another galaxy | |
| NGC 6946 | SAB(rs)cd | Also known as fireworks galaxy for frequent supernovae | |
| Baby Boom Galaxy | Brightest starburst galaxy in distant universe | ||
| Centaurus A | E(p) | Nearest elliptical starburst galaxy. | |
| Large Magellanic Cloud | Being disrupted by the Milky Way | ||
| Haro 11 | Emits Lyman continuum photons | ||
| Sculptor Galaxy | SAB(s)c | Nearest large starburst galaxy | [7] [8] |
| Kiso 5639 | Also known as the 'Skyrocket Galaxy' due to its appearance | [9] [10] | |
| SBS 1415+437 | Blue compact dwarf galaxy | A dwarf galaxy containing a large number of Wolf-Rayet stars | |
| Abell 2744 Y1 | Irregular dwarf galaxy | A very distant galaxy, producing 10 times more stars than the Milky Way |
