University of California, San Diego
Center for Astrophysics & Space Sciences

Gene Smith's Astronomy Tutorial

Starbursts & Colliding Galaxies

Starbursts & Infrared Galaxies

Since the mid-1970s astronomers have known that some galaxies go through intense bursts of star formation, producing large numbers of luminous young stars and related supernovae. Astronomers have dubbed these "Starburst Galaxies". The immensely successful Infrared Astronomical Satellite, launched in 1983, showed that starbursts may be a common mode of evolution for galaxies and that starbursts may be intensely luminous, emitting quasar-like amounts of energy at infrared wavelengths.

Starburst Galaxy Spectrum

The spectrum of a Starburst Galaxy looks much like that of an ionized hydrogen region because the light from these galaxies is dominated by giant HII regions ionized by recently formed, massive, hot stars. Although not all Starbursts may be explained by galaxy collisions, many are clearly stimulated by collisions or mergers of gas-rich galaxies, as in the case of the Antennae (NGC 4038 & 4039) shown below.

Colliding Galaxies NGC 4038 & 4039. The left (B/W) image shows the bodies of the two galaxies with long tails of material which has been stripped from the galaxy centers by gravitational tidal forces. The green outline demarks the Hubble Space Telescope image shown on the left.

This animation of a Galaxy Collision between spiral galaxies was made from a computer simulation, one of the most detailed ever carried out, by Lars Hernquist & Chris Mihos on the UCSD Supercomputer. It was featured in the highly recommended IMAX film, Cosmic Voyage.
Galaxy Merger

Ultraluminous Infrared Galaxies and Quasars





Artists impression of a merger between the Andromeda Galaxy and the Milky Way as viewed from the earth (which may not still be around then). Andromeda and the Galaxy are headed toward each other with a speed of about 120 km/s, suggesting that they may collide in about 5 billion years. As the galaxies near one another Andromeda will grow from the barely visible patch of fuzz to a spindle shaped blob in the northern sky (1). As the galaxies close together tidal forces will distort the disk of our Galaxy and Andromeda (2). As the galaxies begin to collide, the stars will pass between each other (remember, the stars are like grapefruits separated by thousands of miles). But stellar orbits will be distorted and giant, dusty molecular clouds will begin to crash together (3). Collisions of molecular clouds will set off bursts of star formation. The galaxies will pass through each other, but, if the collision is head-on, they will turn around and fall into a common center, perhaps initiating an intense nuclear starburst (4). If this happens, intense supernova explosions and "superwinds" will drive out the gas and dust as stars settle down into new orbits in a newly-formed elliptical galaxy (5). Many astronomers think that the nuclear starburst may feed the Black Holes believed to be lurking in the galaxy nuclei giving birth to a new quasar, as suggested by the jet in the last image.
Artwork by James Gitlin (Space Telescope Science Institute)

Our research suggests that the Luminous IR Galaxy Markarian 231 may house a quasar that was born about 100,000 years ago in a galaxy merger.

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Conducted by Gene Smith, CASS/UCSD.
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Prof. H. E. (Gene) Smith
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Last updated: 29 Sept. 2000