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Object of the Week November 9, 2014 - NGC 520 (Arp 157)
Object data:
NGC 520: RA 01h 24’ 35”, Dec +03 47’ 48”, size 4.5’ x 1.8’, mag 11.6
NGC 520, located halfway between α and δ Piscii, is a superb example of interacting spiral galaxies. Ever since its discovery by William Herschell in 1784, its “abnormal” appearance has puzzled both amateur astro-observers and professional astronomers. Herschell himself described it as “faint, considerably large, extended in PA 133 degrees”. This description fits what would be seen with a small telescope today. On using bigger telescopes, astronomers soon realized that there was quite more than a longish streak of light. In the 1960s the prevalent idea was that NGC 520 was an “exploding galaxy” (somewhat akin to M82), an idea endorsed among others by Edwin Hubble. Research has since proved though that NGC 520 is really a pair of colliding spiral galaxies. The presence of multiple infrared and radio sources from within NGC 520, indicating massive star formation, is further evidence that we are witnessing the collision of two spiral galaxies and, in a sense, we are able to observe in real time what will happen to the Milky Way and Andromeda galaxies in the future from the “safe” distance of approximately 100 million light years. Some researchers believe that the warped shape of the galaxy conglomerate stems from interactions with another neighboring galaxy, UGC 957. With its strange form, especially apparent in deep photos, NGC 520 was an obvious candidate for inclusion in Alton Arp’s Atlas of Peculiar Galaxies, becoming the atlas’ 157th entry (as a "galaxy disturbed with interior absorption").
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Photo of NGC 520 by Adam Block/Mount Lemmon SkyCenter/University of Arizona. Note UGC 957, visible on the lower left hand side.
My own “love affair” with NGC 520 goes way back, to a November 2000 Sky and Telescope article by Richard Jakiel titled “Galaxy Bridges, Tails, and Rings”. Jakiel spends considerable amount of time discussing NGC 520 in the article, describing it as “perhaps the best known peculiar galaxy in the autum evening sky”. A few months after reading the article I tried for the galaxy with my old 10” reflector, but under poor suburban skies I only observed an extremely faint longish patch. After that disappointing experience NGC 520 passed to the back of my mind, waiting for better circumstances.
My interest in NGC 520 was renewed this year though. On one hand I started a systematic approach to observe (at a leisurely pace) as many Arp galaxies as I could, and on the other hand, Uwe Glahn mentioned NGC 520 as an “X-rated galaxy” in the June 15th 2014 OOTW discussion (http://www.deepskyforum.com/showthre...arc”-Arp-199), certainly picquing my interest. So, by August I was ready to tackle NGC 520.
My first attempt on NGC 520 with my 18” was on a very hazy summer dawn in early August, with the galaxy still rather low on the sky. The hazy conditions coupled with the low altitude of NGC 520 gave me an observation quite similar to what I had observed back in 2001. Obviously, this low surface galaxy was a target for steady, transparent nights. My first good observation occurred on September 29th, under the velvety black skies of Lysos, Cyprus (SQM 21.55). At first glance or low magnifications, NGC 520 appeared as a divided “peanut” shaped patch of light. Increasing the magnification to 275x allowed a ton of details to emerge. The galaxy appeared to consist of two distinct parts, divided by a sharp dust lane. The western part was definitely brighter, longer, now more shaped like a banana with a distinctly brighter knot on the northernmost edge. The eastern part was more linear, smaller, fainter, and less well defined, except near the dust lane where it appeared sharp. The southernmost edges of both parts seemed to be slowly fading into southwestern and southeastern extensions respectively, with the northwestern extension longer and more evident. Both parts seemed connected to the north, just before the distinct knot. Although I tried hard, I failed to detect UGC 957, the tiny galaxy north of NGC 520, and I could not detect the “plumes” (so visible in deep photos) flying off the galaxy, even the brighter one to the south. But at the same time, I could readily understand Uhe’s comment about NGC 520 being an “X-rated galaxy”! I was since able to duplicate my observations from my SQM 21.1 backyard several times.
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My observations seemed to be in good agreement with other observations made with dobsonians in the 16” to 20” range. UGC 957 was not seen in any of the observations I managed to locate online. Apparently it must be quite a hard object, requiring considerably more aperture. Alvin Huey, in his “Arp Atlas Observing Guide” mentions spotting an additional dust lane, but I am not sure exactly where he has spotted it using his 22”, perhaps he can enlighten us a bit when he reads this!
What about observations with smaller scopes? Under SQM 21.05 skies at my backyard observatory I was able to see NGC 520 as a very faint, longish patch of light with my 6” f5.9 achromatic reflector, magnifications of 100x to 150x being the best. I believe that NGC 520 should be detectable under good conditions with a four-inch refractor, and I plan to try for it during the coming new moon period. Surprisingly, Skiff and Lunginbuhl mention that the dark lane was detectable with 30cm, so this galaxy should definitely be included as a target even by the owners of relatively small scopes.
As a conclusion, NGC 520 is a very interesting example of the result of the collision of two spiral galaxies, readily visible even in smallish telescopes, and with significant detail apparent in larger scopes. This interactive galaxy pair provides observes with many challenges: What is the smallest aperture to detect it? What is the smallest aperture which shows the dividing dust lane? Can the plumes flying off the galactic conglomerate being detected? How about detecting the elusive companion galaxy, UGC 957?
So, next time you are under dark and steady skies,
“GIVE IT A GO AND LET US KNOW”!
GOOD LUCK AND GREAT VIEWING!
