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Object of the Week September 14, 2014: Arp 86 (NGC 7752 and NGC 7753) and Friends
Object data:
Arp 86
NGC 7753: RA 23h 47’ 05”, Dec +29 29’ 02”, size 3.3’ x 2’, mag 12.8
NGC 7752: RA 23h 46’ 59”, Dec +29 27’ 32”, size 1.0’ x 0.5’, mag 14.3
Arp 86 “Friends”
PGC 72238: RA 23h 43’ 58”, Dec +29 06’ 55”, size 0.9’ x 0.2, mag 16.5
PGC 72239: RA 23h 43’ 57”, Dec +29 07’ 58”, size 0.7’ x 0.3, mag 15.6
PGC 72249: RA 23h 44’ 05”, Dec +29 07’ 15”, size 0.3’ x 0.2, mag 16.7
Ever since I started observational astronomy, I have been fascinated by galaxy pairs, and especially those actively interacting, such as the glorious M51 and NGC 5195. So, when I encountered Arp 86 in Kanipe and Webb’s “The Arp Atlas of Peculiar Galaxies” it immediately grabbed my attention.
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http://ned.ipac.caltech.edu/level5/A...big_arp86.jpeg
Arp 86 is an interacting galaxy pair in eastern Pegasus, one third of the way from Alpheratz (Alpha And) to Scheat (Beta Peg), approximately 270 million light years away. NGC 7753 is a huge, 12th magnitude spiral (classified as SAB (rs) bc), with a smaller, 14th magnitude elliptical galaxy (NGC 7752) at the edge of a long spiral arm. Most photographs do show that the spiral arm leading to the elliptical is definitely distorted due to gravitational attraction, while very deep photographs such as the ones below from Skinakas Observatory at Crete, Greece (http://www.capella-observatory.com/I...xies/Arp86.htm) and Adam Block (http://annesastronomynews.com/annes-...ngc-7753-7752/) show extremely faint streamers of gas, dust, and stars at the edges of the main spiral arms.
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The distorted arms of NGC 7753, along with abnormal rates of star formation in both the galaxies and the connecting “bridge”, indicate that NGC 7752 has already made at least one close pass (and most likely several close passes) very near its larger neighbor, and it is certain that it will be consumed by the larger spiral in the not too distant future (see http://ebooks.cambridge.org/chapter....0511564789A113, http://arxiv.org/abs/1404.7037, and http://www.researchgate.net/publicat...body_modeling).
All photos indicate a very fascinating target for photography, but how does it stand to visual scrutiny? Well, under nice dark skies, both galaxies were readily visible in the 18”, especially at medium to high magnifications. I found that they appeared most detailed at 275X. NGC 7753 appeared as a rather large, elliptical (3’ X 2’) nebulous patch oriented east – west, with a significantly brighter center area, though no distinct nucleus was visible. The extended halo did appear somewhat mottled, hinting that spiral structure could be possibly detected with larger telescopes. NGC 7752 was a small elliptical glow to the south of the 7753, approximately similar in size and orientation to the brighter core of 7753. Surprisingly, although listed as magnitude 14.3, the smaller galaxy was more eye-catching than the larger spiral, presumably because of a higher surface brightness. A single 14th magnitude star appeared between the two galaxies, but alas, no bridge was seen. Despite my trying very hard, I was unable to detect any hints of the beautiful spiral arm connecting the two galaxies, so apparent in photographs of Arp 86.
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After observing this galaxy pair, I checked my books and the web for further visual observations of these galaxies. To my surprise, I discovered that very few visual observations of Arp 86 are available. Alvin Huey (observing with his 22”), David Tosteson (observing with a 25” dob) and Barbara Wilson (observing with a 20” dob) have all observed the mottling of the larger galaxy, but all have failed to detect the arm “connecting” the two galaxies. Maybe someone with a larger scope could possibly detect it. Barbara Wilson mentions “outer arms faintly visible”, I am not though sure if she means the outer, very faint edges of the halo or if she has seen actual arms. Mind you, those of you with smaller scopes should also give it a shot. With my 6” achro under SQM 21.0 skies I could detect an amorphous, faint glow though I couldn’t split the two galaxies at 129X. I had a similar view through an 8” SCT, while Steve Gottlieb has managed to observe both galaxies with a 13.1 dob in the past. I believe that both galaxies should be visible in a 10 to 12 inch class telescope. By the way, three (albeit faint) supernovae have appeared in NGC 7753 over the last few years, so this is another reason for keeping an eye on this galaxy.
As usual, I always like to check if there are further, “hidden” objects near an object of interest, and the vicinity of Arp 86 could not be an exception. Indeed, about 45 arc-minutes west – southwest of Arp 86, I located a trio of 16th magnitude galaxies (PGC 72238, 72239, and 72249) surrounding a (distracting) 13th magnitude field star.
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They were all detected with my 18” dob at 275X and they could all be held steady, so they should be a possible quarry for even smaller scopes. They were all different from each other: PGC 72238 appeared quite longish, PGC 72239 had a more roundish elliptical shape, and PGC 72249 was round with a brighter center. Has anybody observed these little ghosts?
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So, next time you are in Pegasus, remember to stop by Arp 86 and visit along with its friends. And as always,
“GIVE IT A GO AND LET US KNOW!”. Good luck and Great viewing!
