May 7, 2013
Arp 81, interacting galaxies in Draco
Image Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and W. Keel (University of Alabama, Tuscaloosa)
Arp 81 (also designated UGC 11175 and VV 247) is a strongly interacting pair of spiral galaxies, consisting of NGC 6621 (right) and NGC 6622 (left) seen about 100 million years after their closest approach. The galaxy pair is located about 277 million light-years away in the constellation of Draco and is speeding away from us at around 6330 kilometers per second.
The name “Arp 81” derives from being included in Arp’s Atlas of Peculiar Galaxies, a catalog of 338 peculiar galaxies, drawn up by Halton Arp in the years from 1962 through 1967 and published by the California Institute of Technology.
Both galaxies show tidal effects, especially NGC 6621, the larger of the two, which shows twisted streams of gas and dust, and a long tidal tail of stars, stretching for some 200,000 light-years that has now been wrapped behind its body.
The collision has also triggered extensive star formation between the two galaxies. Clusters of stars can form quickly after a strong enough perturbation, and Arp 81 has indeed a very rich collection of young massive star clusters (an even richer collection of star clusters than the well know Antennae galaxies).
The bright bluish double knot seen in the overlap region between the galaxies is a 13,000 light-years wide massive starburst of young hot stars induced by the collision. The pink knot near the nucleus of NGC 6621 is very large emission nebula, perhaps in a pre-starburst stage. The galaxy pair is classified as a starburst system and is as such a strong far-infrared and radio source.
The galaxies are in their late interaction phase and are destined to merge into one large galaxy in the distant future, making repeated approaches until they finally coalesce.
This composite image is taken on March 15, 1999 with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope using two different color filters, for red and for blue.