NGC 5216: Keenan's System by Winder/Hager

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

Article Updated: 26 Apr , 2016


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.”

Plate 3: Zwicky - Palomar Observatory courtesy of CaltechHowever, 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!

21 Responses

  1. Mark says:

    IF I recall correctly, Arp contends that redshift is not an indication of recession speed or distance from earth but is caused by something else. Part of his evidence was interacting galaxies which had widely different redshifts. This meant that the galaxies had to be close to each other, but their different redshifts would indicate that they are far away from each other. Since their interaction meant that they really were close to each other, this meant that the redshift = distance interpretation must be wrong. Those who disputed Arp would question whether or not the galaxies were really interacting or whether this was a superimposed illusion. It sounds like it is being freely acknowledged that Arp was right about at least some of his interacting galaxies. Am I reading this right? If so, doesn’t that mean that Arp is right about redshift not being an indicator of distance?

  2. Wooo hooo! I just love it when readers are sharp… πŸ˜€

    Damn fine reasoning, Mark. Tests for large-scale systematic errors in maps of galactic reddening have also occurred concerning this particular Arp pair and they were included in a survey of galaxy redshifts in 1983. From what I can gather of the Tonry model, radial velocity redshift measurements can be impacted by gravitational pull between galaxies as well.

    As for Arp being right? Yep. This is also my opinion. Redshift is not the know all and end all indicator of distance!

  3. Aodhhan says:

    Bravo Mark!

    In all due respect to Mr. Hubble, I too question the doppler red shift on objects when it comes to the “Big Picture”. Especially when it takes a star moving approximately 1900 miles/second to change just 1% in the frequency of light. As red as some of the furthest galaxies (from us) are, they would have to be moving away at blinding speeds.

    There are a lot of different thoughts on different things with this subject. Coherent Raman Interactions to name one.
    I’ll shut up there, because I can ramble all day about this!

    Therefore, there must be something else which is stretching the wavelengths (or broadening of spectral lines if you prefer) over time and or distance. Figure it out, and you’ll probably find the Nobel Prize for physics on your mantle!

    Great choice for an article Tammy!

  4. Mark says:

    Tammy thanks for the prompt response on a controversial subject (a rarity)!

    I’m curious … do you have any idea if the majority of astronomers still have doubts about whether the galaxies in Keenan’s system are really interacting? If they don’t have doubts, how do they maintain their redshift = distance theory? (As far as I know Arp is still generally viewed as an outcast … am wrong about that?)

    Thanks again for responding!

  5. Bill says:

    It’s good to see alternatives to the widely accepted interpretation of redshift in a main article. Kudos to UT for allowing alternative views in the articles and for allowing the free-ranging discussions.

  6. If anyone still has doubts, they really need to look a Zwicky’s work. He stated that the rediscovery of the pair caused some “merry confusion” and then set to work trying to solve the riddle. One thing all three astronomers shared (Keenan, Arp and Zwicky) was they were all somewhat outcasts – but who can deny what you see when you look at Zwicky’s photographic plate? (Now added.) He was quick to point out: ” It is likewise a common feature of many connected double nebulae that only one of them shows a distinct countertide.”

    The very coolest part is that another Arp pair of interacting galaxies – Arp 285 (NGC 2856/4), have also been studied (Beverly Smith et. al) just a couple of months ago for the same basic phenomena. “This system contains a striking example of “beads on a string”: a series of star-formation complexes ~1 kpc apart. These “beads” are found in a tail-like feature that is perpendicular to the disk of NGC 2856, which implies that it was formed from material accreted from the companion NGC 2854… 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… this feature is part of a expanding ripple-like “arc” created by an off-center ring-galaxy-like collision between the two disks.”

    If this can occur in one Arp pair – why not another? After all, Zwicky himself states, “As in all other cases, it will be worthwhile to make further attempts at obtaining spectrograms of the faint luminous intergalactic formations in Keenan’s system. So far it can only be said that these appear blue and show no emission lines, from which fact we conclude that they consist of stars.”

  7. joe says:

    So it black holes can keep light from escaping, and they spaghettize everything at the event horizon, why couldn’t blackholes stretch light wavelength?

    The larger the black hole the greater the redshift? Is that possible?

  8. H-town Mack says:

    So, I’m too late and Megatron has already created a spacebridge there! Seriously, I wonder if ET’s have figured out a way to use that “bridge” to some effect. As in bridging the distance in less time than normal.

  9. John Mendenhall says:

    Nowhere in this article are the redshifts of the component galaxies mentioned. Would any of the (real) astronomers out there care to enlighten us?

  10. Timber says:

    something of an aside, several years ago Halton Arp made a presentation at either the Winter Star Party or the Texas Star Party (can’t remember which) at which time i bought one of his DVD’s, about half of which is in Danish (?). Is anyone familiar with his DVD and might know of a translation somewhere and if it might be available? Thanks

  11. Bill says:

    John, I found

    NGC5216 z = .0979 from

    NGC5218 z = 0.009643 from…387…40M&NGC+5218

    I would have preferred these from a common source. The SIMBAD link did not have 5216.

    An interesting contrast, if these sources are correct.

    And apologies, I am but an imaginary astronomer.

  12. Bill says:

    John, Maybe my post with the source links will show up after a moderator clears it. I found the redshifts:

    NGC5216 z = .0979
    NGC5218 z = .009643

    I was careful with the decimal places.

  13. Tyler Durden says:

    Good news for inhabits on 5216 or 5218. They can easily colonize two galaxies for the price of one by sending waves of colony ships along the bridge.

  14. ScepticTim says:

    Great article Tammy. I wonder if the red shift difference here can be attributed to differences in velocity of the two galaxies due to either their orbiting, after interaction, about a common centre of mass or some other change in their original trajectories, causing one of the pair to accelerate towards us while the other has been accelerated away?

  15. Jon Hanford says:

    Two recent papers on Arp 104 may be of particular interest here: arXiv:astro-ph/0701452 (2007 referencing CO & HI studies) and arXiv:astro-ph/0605015 (2006 referencing imaging & spectra w-‘cross-fueling’ occuring). In the second paper by Roche, SDSS redshifts are given for: NGC 5218 z=0.009783 (2933km/s) & NGC 5216 z=0.009804 (2939km/s). I, too, was careful with the decimal places. The second paper also concurs with Tammy’s statement that the blue bridge between the galaxies is composed of young, blue stars 200-400 My of age. This system is one of my favorite interacting galaxy pairs and I complement Tammy for writing up this informative, thought-provoking article. Hope to see more like it in the future.

  16. Jon Hanford says:

    The paper referenced by Tammy in this article by Beverly Smith is I believe arXiv:astro-ph/0610562 (2006). Links to all 3 papers I referenced can be found at the Nasa Extragalactic Database (NED) & the SIMBAD database. Including links to some of these papers in future articles would be greatly appreciated & may help to clear up or detail issues mentioned in some articles (or not, depending on the reader). Anyway, thanks again for articles of this type.

  17. ScepticTim says:

    My thanks to Jon Hanford. A quick scan of the papers that you referenced solves the supposed red shift discrepancy and eliminates the need to explain the apparent differing velocities of the galaxies.

  18. Tom says:

    Are there any theories as to what these bridges are composed of? What could be the reason for the pulling of matter across the expanses of space!? This is just baffling to me!

  19. Jon Hanford says:

    According to the paper by Roche(2006), the bridge “(has) the blue colours of stars 0.2-0.4 Gyr old stars” . Earlier VLA radio observations of the bridge saw substantial amounts of HI (neutral hydrogen) gas. Gravitational attraction between the two galaxies has pulled this bridge of hydrogen away from both galaxies & it seems that conditions are favorable for star formation from this raw material. Spitzer infrared images show only weak emission from dust in the bridge, indicating low dust content. Also, visible light spectra of the bridge show no emission lines in the bridge, so little star formation is currently taking place here. In the future, the bridge may fragment into Tidal Dwarf Galaxies (TDGs) or fall back into the gravitational grips of the two galaxies. Under the right conditions NGC 5216 & 5218 may stay gravitationally bound and coalesce into a single (elliptical) galaxy far into the future. But just think of the view from a planet orbiting a star in the bridge between the two galaxies!

  20. Joe Schomer says:

    The high horsepower engine of real science is healthy scepticism requiring infinite doubt and scrutiny. There’s always something missing. Detective Colombo always has just one more question. The ultimate democratic process of discovery.
    There are Newtonian Apples constantly falling everywhere and curious young Newtons observing and speculating. What if the classic Hubble Red Shift falls to something nobody ever considered? It’ll make a strange universe even stranger. Consider quiet shy Einstein working at a dull job in a patent clerk’s office pondering space and time. Somewhere, as we speak, there’s some serious off the wall intuitive thinking occurring that could will change EVERYTHING!
    (Carlos Castenada called that instructive inner voice ‘the whispering of the Nagual.’)

  21. i loved the works of castenada… πŸ˜‰

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