Bridge Across Space: “Keenan’s System” by Martin Winder and Dietmar Hager

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Take a very close look at this image of NGC 5216 and companion galaxy NGC 5218 and you’ll see a bridge of galactic material that joins these two isolated galaxies. Located in the constellation of Ursa Major (RA 12 30 30 Dec +62 59), this tidally connected pair known as Keenan’s System has been well-studied but you’ll find they have rarely been imaged.

First discovered by Friedrich Wilhelm Herschel in 1790 and later studied as Intergalactic Nebulae in 1926 by Edwin Hubble, it wasn’t until 1935 until PC Keenan noted this double galaxy mystery seemed to be connected by “luminous debris” – a connection that spans 22,000 light years. Keenan noted the peculiar structure in his paper but it would be 1958 before the bridge of material was “rediscovered” by observers at Lick and Palomar observatories in “The Interaction of Galaxies and the Nature of Their Arms, Spanning Filaments and Tails”.

By 1966, peculiar type spiral NGC 5216 and the globular galaxy NGC 5218 were included as Arp 104 into Halton Arp’s Catalog of Peculiar Galaxies and the 17.3 million light year distant pair were beginning to capture the attention they deserved. Studies were conducted of active galactic nuclei among interacting galaxies and galaxies with extreme tidal distortions and it wasn’t long before science realized these two galaxies had collided – stripping stars, gas and dust from each other which appear about them like skewed halos. Once interaction has occurred, the bridge between them fills with “stars in new and perturbed orbits”.

In infrared studies done by Bushouse (et al), even more fascinating details have been revealed as we learn that galaxy-to-galaxy collisions can produce higher infrared emissions. “Only the most strongly interacting systems in the sample show extreme values of infrared excess, suggesting that deep, interpenetrating collisions are necessary to drive infrared emission to extreme levels. Comparisons with optical indicators of star formation show that infrared excess and color temperatures correlate with the level of star-formation activity in the interacting galaxies. All interacting galaxies in our sample that exhibit an infrared excess and have higher than normal color temperatures also have optical indicators of high levels of star formation. It is not necessary to invoke processes other than star formation to account for the enhanced infrared luminosity in this sample of interacting galaxies.”

What’s happening between the pair is causing starburst activity, perhaps from the sharing of gases. According to Casaola (et al); “From the data it appears that interacting galaxies have a higher gas content than normal ones. Galaxies classified as ellipticals have both a dust and gas content one order of magnitude higher than normal. Spirals have in most part a normal dust and HI content but an higher molecular gas mass. The X-ray luminosity also appears higher than that of normal galaxies of same morphological type, both including or excluding AGNs. We considered the alternative possibilities that the molecular gas excess may derive from the existence of tidal torques which produce gas infall from the surrounding regions… it appears that interacting galaxies possess a higher molecular mass than normal galaxies but with a similar star formation efficiency.”

However, the single most interesting point is the remarkable filament which connects NGC 5216 and companion galaxy NGC 5218 – a “concentrated string-like formation connecting the two systems and the fingerlike extension, or countertide, protruding from the globular cluster NGC 518 and starting on the same tangent as the interconnecting filament.” It was this very string of material which has been a very recent study of Beverly Smith (et al) in the Spitzer infrared, Galaxy Evolution Explorer UV, Sloan Digitized Sky Survey and Southeastern Association for Research in Astronomy. Their studies helped to reveal these “beads on a string”: a series of star-formation complexes. According to their findings; “Our model suggests that bridge material falling into the potential of the companion overshoots the companion. The gas then piles up at apogalacticon before falling back onto the companion, and star formation occurs in the pile-up.”

The light data for this awesome image was gathered by AORAIA member Martin Winder and processed by Dr. Dietmar Hager. This particular image took nearly 10 hours of exposure time and untold hours of processing to turn it into the beautiful, study-grade photo you see here. We thank Mr. Winder and Dr. Hager for sharing this exclusive photo with us!