Image of the Day: “The Cosmic Eyes”


Resembling a pair of all-seeing cosmic eyes, NGC 6621 and 6621 are a strongly interacting pair of galaxies, seen about 100 million years after their closest approach. It consists of NGC 6621 (to the left) and NGC 6622 (to the right). NGC 6621 is the larger of the two, and is a very disturbed spiral galaxy. The encounter has pulled a long tail out of NGC 6621 that has now wrapped behind its body. 

The collision has also triggered extensive star formation between the two galaxies. Scientists believe that Arp 81 has a richer collection of young massive star clusters than the notable Antennae galaxies (which are much closer than Arp 81). The pair is located in the constellation of Draco, approximately 300 million light-years away from Earth. 

NGC 6622 passed just at the edge of the disk of NGC 6621 in an orbit nearly parallel to the disk plane. In addition to the tidal distortion that has pulled the long tail out and wrapped it behind the main body from our vantage point, the encounter has triggered extensive star formation; this system has a richer collection of young massive star clusters than the better-known and closer Antennae, NGC 4038/9. 

It is interesting that the compact collection of star clusters and emission nebulae between the two galaxy nuclei coincides with a region in which the dynamics of the disk are so profoundly disturbed that the slope of the velocity field changes sign, which may implicate gravitational instability within the disk (a result of tidal disturbance) as one cause of formation of such massive star clusters.  

Galaxy clusters can form quickly after a strong enough perturbation, not awaiting further churning as the galaxies come together and merge later. NGC 6622  is not escaping this encounter without change either. While it has no apparent star formation now, the corkscrew form of the dust lane crossing in front of it shows that the gas and dust are being strongly affected by its gravity and some will likely end up being transferred to the smaller galaxy, perhaps to fuel a much later episode of star formation and nuclear activity. 

The Dali Galaxy via NASA and


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