Starburst galaxy

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. 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.

Definition

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" b. Generally, a birthrate parameter ≥ 3 is used as the cutoff for starbursts. 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 > 10 9.2 M ⨀ {\displaystyle 10^{9.2}M_{\bigodot }} ), this may be an overestimation.

Triggering mechanisms 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.

Types

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:

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