This is the interesting story of NGC 450 / UGC 807.
Please do me a favor. Before reading the rest of this description, please examine the image carefully now, so that you don’t have any of the information below to bias you. Look especially at the intersection/overlap of the 2 galaxies.
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NGC 450 is located 60 million light years away in the constellation of Cetus at a declination of -1 degrees. It is a magnitude 11.8 galaxy which spans 2.8 arc-minutes in our apparent view. This corresponds to a diameter of 50,000 light years. Notice the bright blue HII regions in the left side of the NGC 450 disk.
The yellow companion at upper left is UGC 807.
To me, at first glance, these two galaxies looked unrelated, with no deformations or star streams. The color and structure of UGC 807 suggests that it is a distant, larger galaxy. And indeed, the redshift data indicates that UGC 807 is almost 10 times further away, at a distance of 0.5 billion light years. At this distance, UGC 807 is a Milky-Way-sized galaxy at 120,000 light years in diameter. The Hubble image (see Revision E) of this galaxy also seems to confirm the above.
So far, so good.
Here is where it gets interesting.
As many of you know, Dr. Halton Arp studied galaxies extensively throughout his life and questioned the theory of cosmological redshift. Key to his argument was his identification of pairs of objects from optical images that appear to be connected but that have much different redshifts. The best known example of this is the pair
NGC 4319 / Markarian 205. But, unknown to me until today, another closely studied pair was this object, NGC 450 / UGC 807.
After corresponding with Dr. Arp in 1980 about this pair, detailed work resulted in the publication of a paper in 1983 in the Astrophysical Journal by Dr. Rubin and Dr. Ford entitled, “The Noninteracting Spiral Pair, NGC 450/UGC 807”. The paper concluded from the rotational velocities that these 2 galaxies were unrelated and that ”the pair cannot be used as evidence for non-cosmological redshifts”.
Obviously, Dr. Arp was not happy with this publication. Later, in 1994, he published a paper in the Astrophysical Journal with astronomers from Spain entitled, “Testing for Interaction Between the Galaxies NGC 450 and UGC 807”. Getting this paper published was a long road. According to Dr. Arp, when the paper was submitted for publication “it elicited furious reactions by 2 referees in a row”. Also according to Dr. Arp, the principal author of the paper (Dr. Moles) “was so appalled (by the rejections) that he considered giving up research”. But eventually they found a 3rd referee who supported the paper, and it was published. Dr. Arp was disappointed that “no notice was taken” by the astronomy community when this work was finally published.
The paper concludes that “one would have to invoke an enormous conspiracy of accidents in order to avoid the conclusion the UGC 807 is a moderately low luminosity galaxy interacting with NGC 450”. Core to this conclusion are 2 visual clues:
- The bright blue HII regions in the side of the disk closest to UGC 807. In his Seeing Red book, Dr. Arp says that “These glowing regions of excited hydrogen gas are so exceptional that I frankly cannot see how anyone with reasonable common sense and good judgement would not immediately realize that they are a result of the unusually close interaction with the companion."
- The “short luminous tail” of UGC 807, on its lower right side where it overlaps with NGC 450. The paper says that “the detached arm around the small galaxy has the same colors as the main object and therefore is made up of the same material”.
The above is a fascinating example of how different people, including really smart people, can interpret visual information differently, not just at first glance, but after many years of study.
I respect Dr. Arp and his body of work, but I remain unconvinced that these 2 galaxies are interacting.
What do you think?
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