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Object of the Week 21 June 2015: The Draco Triplet (and Friends).
OOTW 21 June 2015: The Draco Triplet (and Friends)
Object data:
NGC 5981: RA 15h 38', Dec +59 23', size 2.8’ x 0.5’, mag 13.6
NGC 5982: RA 15h 39', Dec +59 21', size 2.6’ x 1.9’, mag 11.5
NGC 5985: RA 15h 40', Dec +59 19', size 5.5’ x 3.0’, mag 11.5
Friends:
NGC 5976: RA 15h 37', Dec +59 24', size 0.9’ x 0.4’, mag 15.5
UGC 9934 (=NGC 5976A) RA 15h 36', Dec +59 34', size 1.1’ x 1.0’, mag 15.1
Three months ago, through this great OOTW series, I highlighted the very well known Leo Triplet. Today we will explore another triplet in Draco, which although less well known, it is a quite spectacular target.
The Draco Triplet consists of three galaxies, NGC 5981, 5982, and 5985, all of which lie approximately 120 million light years from earth. Despite their similar distances and their nearness to each other, it is not certain that they interact with each other or that they are gravitationally bound. Still, there are quite a bit of astrophysical questions regarding each galaxy. NGC 5981 is an edge-on barred spiral system with evidence of a fore-shortened stellar disk. NGC 5982 is an elliptical system that apparently has recently undergone a minor merger, resulting in a distinct dust halo. Also, NGC 5982 is suspected of harboring a black hole in its center. Finally, NGC 5985 is another barred spiral classified as a Seyfert galaxy, also harboring a black hole in its core. Sure enough, all three galaxies are nice candidates for astrophysical research. But now it’s time to concentrate on the Draco Triplet from the deep sky observer’s point of view.
DSS image of the Draco Triplet:
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Draco Triplet image by Capella Observatory (via Skinakas Observatory, Crete), http://www.capella-observatory.com/I...es/NGC5985.htm
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Unlike the Leo Triplet which is a target for everything from binoculars to large dobs, the Draco Triplet is more suited to medium and large scopes. Athough they should all be visible under very dark skies in a 12 inch, it really takes aperture to fully appreciate the three galaxies, which all lie in a 30 arc-minute east to west line. But even those with smaller scopes should not despair, as we shall see you can observe parts of the group even in a four-incher.
With my 18 inch I could readily see all three galaxies in the wide field of view of my 21mm Ethos, and they were set in a very beautiful starry background. 12th magnitude NGC 5985 appears as a bright but diffuse, low surface brightness oval with a slightly brighter center in the eastern part of the field. NGC 5982, the central galaxy, also of magnitude 12, is the catchiest of the three galaxies, sporting a star-like nucleus in a bright diffuse halo. NGC 5981, the westernmost galaxy, is fainter at magnitude 13.6 and appears as a faint thin streak of even light. Increasing the magnification to 159X with the 13mm Ethos concentrates the observer’s attention to the three galaxies. Now, some mottling appears on the broad glow that is NGC 5985, while the thin sliver of NGC 5981 appears more distinct.
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At high power each galaxy shows distinct features. At 275X NGC 5985 appears distinctly mottled, suggesting the presence of spiral arms. Actually, the eastern edge of the galaxy appears decidedly brighter, while a tiny brightening appears on the galaxy’s southern edge. Also, the southern part of the galaxy appears to fade more slowly into the sky background, somehow appearing that the galaxy is larger south of the core. All three features are confirmed on photographs of the galaxy. The easternmost arm of the galaxy appears to be brighter than the rest and this causes the brightening of the eastern edge, while a faint star is superimposed on 5985’s southernmost arm. Finally, the southern part of 5985 is indeed less well defined and appears larger even on photos – amazingly all these details can be observed at high powers with my 18 inches, and I am pretty sure they may be detectable in even smaller apertures.
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Using a high power of 275X, the central galaxy, NGC 5982, initially looks exactly like it did at lower powers. A more detailed look though reveals that this elliptical has a three-layered structure: A very bright, tiny but not quite star-like nucleus, a broad, relatively bright halo extending for a few arc-minutes, and then a diffuse, rather faint outer halo that slowly blends into the sky background.
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The edge-on spiral NGC 5981 is yet a different story. This galaxy appears significantly fainter than the other two (my notes indicate that I did not see it with my old 12-inch dob from a rural area back in 2009, although Steve Gottlieb describes it as faint through a 13” dob in the NGC/IC Project home page). At a magnification of 275X it has a distinct cigar shape with a slight central brightening. With averted vision I could detect a slight shadow flickering on and off covering a small part of the center of the galaxy near the eastern edge. With considerable effort I managed to keep the flickering shadow steady enough, and it does correspond nicely with the small dust lane that exists on NGC 5981 photographs. I could not find any mention of observations of NGC 5981’s dust lane, with the sole exception of a comment by Owen Brazell who mentions (in the Webb’s Deep Sky Society June 2012 galaxy of the month section) that “It may be possible to see the dust lane with large telescopes”. Certainly it should be easier with scopes in the 25-inch and up class.
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As usual, after observing the trio, I did check to see if any other possible interesting targets lie nearby. Several mostly faint and tiny galaxies are present, but after checking photos I found two that could be possible targets. The first is 15th magnitude NGC 5976, located about 8 arc-minutes west of NGC 5981. I had to use 275X to detect it as a very small, very faint, elongated 2:1 smudge. Another 10 arc-minutes to the west lies another 15th magnitude galaxy, UGC 9934, sometimes known as “NGC 5976A”. At 275X I could just detect a roundish tiny fuzz at the correct position, holding it steady with averted vision and occasionally seeing it with direct vision as well. I was very surprised not to find even a single visual observation of this galaxy in any observing guide or in the internet. Even Steve Gottlieb in his NGC 5976 entry in the NHC/IC Project page mentions about it (as NGC 5976A) “not attempted”. Anyway, under SQM 21.1 skies it is certainly visible in 18” and under darker skies I believe it might be doable with 14-15” scopes!
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What about those with smaller scopes? With my 6-inch achro at 129X I could readily see NGC 5982 as a little diffuse ball of light with a bright center, while NGC 5985 appeared as a very faint diffuse oval of brightness with an ever-so-slightly brighter center. The galaxy seemed to expand with averted vision. I did try hard for NGC 5981 but I was unable to really detect its presence.
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Using my 4” Takahashi refractor at 109X I again detected easily NGC 5982 appearing more or less as an out-of-focus star. NGC 5985 though could not be seen with any certainty even with averted vision. At times I thought I could glimpse something at the correct position with averted vision, but I could not hold it steady enough to really confirm its presence, so I will say that NGC 5985 was just “suspected”. But I do believe that under very dark skies an experienced observer might be able to see 5985 even in a four-inch refractor. I plan to try again with my 4” in July from SQM 21.6 skies to see if I can confirm its presence.
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Thus, many challenges exist for those who decide to tackle the “other” Triplet in Draco. Those with large scopes can spend many hours exploring the peculiarities of each galaxy in the trio, as well as trying to see other faint galaxies nearby. For those with medium sized (8-13”) scopes, the first challenge will be to detect NGC 5981, and then to extract details out of the other two trio members. Finally, those with scopes in the 4-6 inch range have the challenge of seeing anything more than NGC 5982, and pushing their scope (and eye) capabilities to the limit in bringing out details from the galaxies they can see.
So, during those nice summer evenings, turn your scopes to the Draco Triplet, and…
"Give it a go and let us know!
Good luck and great viewing!"
