From CFHT's Astronomy Picture of the Week
Collisions between companion galaxies are not unusual. Indeed, the average distance between two galaxies in the intergalactic medium is such that mutual encounters are probable, especially in groups and clusters. Collisions were even more frequent in the past when the Universe was younger and smaller. Recently it has been found that galaxies are subject to a strong evolution due to their environment, and collisions play a major role in this process. Galaxy interactions cause major morphological transformations, trigger violent starbursts and in general contribute to strengthen the exchanges between the interstellar and intergalactic medium.
This image presents an example of a spiral galaxy being torn apart by another elliptical galaxy. Both galaxies are located close to the center of a cluster of galaxies, Abell 1185. Number 105 in the Atlas of Peculiar Galaxies compiled by H. Arp, this system has been nicknamed "The Guitar", because of its suggestive morphology. The spectacular jet-like tails that emanate from the galaxies are intriguing but rather common in these interacting systems. They mainly consist of stars that have been pulled out from the disks (more or less the galaxies' mid-planes) of the parent galaxy through a purely gravitational, tidal, effect. Before their final merging, colliding galaxies may hence loose part of their stellar and gaseous content. This material might afterwards recondense again in the intergalactic medium and collapse to form second-generation galaxies. Two such newly born baby galaxies can be seen in Arp 105 at the extremity of each tidal tail, more than one hundred thousand parsecs (325000 light years) from their parents for one of them. They can be seen at the top and bottom of this image. Their properties are very similar to classical dwarf galaxies. The northern object (at the top of the image) is as luminous as the Magellanic Clouds.
A complementary study has shown that both "tidal dwarf galaxies" contain massive clouds of atomic hydrogen which has also been expelled from the disk of their parents. This gas reservoir is recycled in the tidal dwarf galaxies to form new generation of stars. The compact knots in the tails of Arp 105 are such star-forming regions.
Image Title: Arp 105: The Guitar
Credit: Image courtesy of P.A. Duc Service D'Astrophysique, DAPNIA/DASM/CEA Saclay, France
Text based on accompanying on-line materials.
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Last Modified On: Saturday, December 16, 2000