Image Credit & Copyright: Robert Gendler (http://www.robgendlerastropics.com/)
The Antennae Galaxies [NGC 4038 (right in this image) and NGC 4039] are a pair of merging galaxies of some 61,000 light years across (without the tidal tails), located about 45 million light-years away within the NGC 4038 group of galaxies in the southern constellation Corvus (and also visible very low on the southern horizon in the Northern Hemisphere). The Antennae are among the closest known merging galaxies, while they are moving away from us at about 1642 kilometers per second.
The Antennae are in the midst of a galactic collision. About 1.2 billion years ago, the Antennae were two separate galaxies. NGC 4039, the larger of the two before they collided, was an unbarred spiral galaxy and NGC 4038 was a barred spiral galaxy. 900 million years ago, the Antennae began to approach one another, and 600 million years ago, the Antennae passed through each other. 300 million years ago, the Antennae’s stars began to be released from both galaxies. Today the two tidal tails of ejected stars, gas and dust extend out almost 500,000 light years from their respective centers, making them resemble the antennae of an insect, hence the name of the galaxy pair.
Within 400 million years, the Antennae’s nuclei will collide and become a single core with stars, gas, and dust around it. Observations and simulations of colliding galaxies suggest that the Antennae Galaxies will eventually form one giant elliptical galaxy. They give us a preview of what may happen when our Milky Way collides with the neighboring Andromeda galaxy in several billion years.
The galaxies are criss-crossed by filaments of dark brown dust, and dotted by over a thousand bright star clusters, containing tens of thousands of massive, hot, young stars — the result of a burst of star formation triggered by the collision. The brightest and most compact of them are called super star clusters.
Only about 10% of the newly formed super star clusters in the Antennae will live to see their ten millionth birthday. The vast majority of the super star clusters formed during this interaction will disperse, with the individual stars becoming part of the smooth background of the galaxy. It is however believed that about a hundred of the most massive clusters will survive to form regular globular clusters.
The most massive of the massive young stars have already sped through their evolution in a few million years and exploded as supernovae. Two supernovae have been discovered in the Antennae: SN 2004GT and SN 2007sr. The two small, thin lines in this image pinpoint the location of SN 2007sr, a Type Ia supernova that occurred on December 18th, 2007.
A Type Ia supernova is a result from the violent explosion of a white dwarf star. This category of supernovae produces consistent peak luminosity. The stability of this luminosity allows these supernovae to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.
Supernova explosions are enriching the intergalactic gas with elements like oxygen, iron, and silicon that will be incorporated into new generations of stars and planets. In the Antennae, there are also rich deposits of neon, magnesium, and silicon discovered, which are among the elements that form the building blocks for habitable planets.
Although galaxy mergers today are not common, it is believed that in the past they were an important channel of galaxy evolution. Therefore understanding the (physics of) galaxy mergers is a very important task for astrophysicists and cosmologists.