OOTW August 16th, 2015: The Dolphin Triplet of Galaxies: NGC 6927, NGC 6928, and NGC 6930

The Dolphin Triplet: NGC 6927, 6928, and 6930.

NGC 6927: RA 20h 33', Dec +09 55’, size 0.9’ x 0.4’, mag 14.4
NGC 6928: RA 20h 33', Dec +09 56', size 2.0’ x 0.6’, mag 12.2
NGC 6930: RA 20h 33', Dec +09 52', size 1.1’ x 0.5’, mag 12.8

Μy last two OOTW have dealt with galaxy triplets (the Leo and Draco triplets). So I decided to create a trilogy out of the "Triplet" theme, concluding with a galaxy triplet in an unexpected constellation: Delphinus, the Dolphin.

Delphinus is a summer constellation normally associated with double stars and globular clusters, not galaxies. Yet it does harbor some interesting galaxies, with NGC 6927, 6928, and 6930 making a nice triplet. Unlike though the other triplets in our “trilogy”, the Dolphin Triplet requires really dark skies and medium to large scopes to really appreciate. All three galaxies were discovered exactly 152 years ago (on August 15th, 1863) by A. Marth using the 48” speculum mirror he installed on the (nearby for me!) Mediterranean island of Malta. Amazingly, practically nothing is known (or at least readily available) regarding the astrophysics of these galaxies. NGC 6930 has been the host of supernova 2003gt, reaching 16th magnitude, but that’s about all. I suspect that NGC 6928 and 6930 (both spirals of SbB type) might well be near each other in space as they appear quite similar in a telescope, but then NGC 6927 is an SO galaxy so it could also be near the other two.

Attachment 1754

Observationally, NGC 6928 and 6930 are the two prominent galaxies, while all three can fit in a 20 arc-minute field of view. NGC 6928 is the brightest at magnitude 12.2, while NGC 6930 is a bit fainter at 12.8, and tiny 6927 is fainter yet at magnitude 14.4. Note that some fainter galaxies do appear in the Deep Sky Survey image, but they were too faint to be detected by my 18”.

In my 18” Obsession at 275x, both NGC 6928 and 6930 appear as elongated (3:1) ellipses, with 6928 sporting a rather bright core. NGC 6930 appears more diffuse, with no distinct central brightening. The southernmost edge of 6930 appears to almost touch the north apex of a triangle made of 12th magnitude stars. NGC 6927 appears as a much fainter, small oval shaped blur with a perceptibly brighter stellar core. During moments of good seeing I could see that the outer part of 6927 appeared larger, but for the most part it was quite small (but could be held steadily with direct vision). NGC 6927 was not visible at lower magnifications.

Attachment 1755Attachment 1756

Both NGC 6928 and 6930 may be seen with medium sized scopes. Steve Gottlieb managed to observe both in his 13” scope, though he mentions that both were quite faint. Owen Brazell in the Webb Deep-Sky Society “Galaxy of the Month” August 2013 article mentions that NGC 6928 may be seen with 22cm (8.5 inches) and again Steve Gottlieb has an observation with an 8” scope in which he mentions “averted vision only”. I tried to observe these galaxies using my 6” achromatic refractor from my SQM 21.1 backyard skies, and at times I did suspect “something” at the correct position, but I could not hold it steady enough (even with averted vision) to draw it. So it appears that although these galaxies are really more suitable for telescopes of 10 or more inches, under very dark and clear skies it might be possible to detect at least one with even 6 inches.

So, during the next warm summer evening you are out observing, swing your scopes to Delphinus, and…

"Give it a go and let us know!
Good luck and great viewing!"

Interesting field. Here is another one like that in Delphinus, KTG 71 (16", 225x, 60 deg. AFOV, south up - NGC 6956, UGC 11620, UGC 11623 at ~190 Mly):

http://1.bp.blogspot.com/-dzNMn1Gu32...+UGC+11623.jpg

Gonna give this a go in Germany next New Moon. But only with good transparency and seeing ('cause I use a 10")

I seem to remember having seen these galaxies with my 6" telescope from the Netherlands in the late 90s or early 00s. I am sure it was a triplet of galaxies in Delphinus but I cannot find any notes of the observation. Back then I was hardly keeping track of my observations :(

Very nice group Rolandos, which shows some interesting details like the twisted elongated center of NGC 6928 or the near companion of NGC 6930 which was both barely visible in my old 16" under very good transparency and seeing.

16", 225x, NELM 7m+
Attachment 1757

I've observed the brighter two (6928/30) of these several times with a 12.5" under yellow- or green-zone skies, using the TriAtlas 'C' chart to starhop there from NGC 6934. -28 and -30 were fairly difficult objects under typical southern Illinois observing conditions; now that I'm in Oregon, I'll have to try them in the better seeing and darker skies.

Two challenges in this group for 18" and larger scopes are the companion at the north end of NGC 6930 (known as 2MFGC 15598 = PGC 200365) and the small galaxy 2' south of NGC 6927 (known as NGC 6927A = PGC 64924). I found these both tough objects in my 18-inch (intermittently visible with averted vision), though Uwe was also barely successful with a 16" in very good transparency (NELM 7m+ skies) and seeing. Finally, there is 2MASX J20330454+0951338 = PGC 214749, situated 1.7' southeast of NGC 6930. I missed this one, so I'll give it a try next month.

I tried this group two nights ago at Okie-Tex star party. I completely missed NGC 6927 because I wasn't looking for it. But as for the companion of NGC 6930, here's what I have:

"Far averted vision revealed the length of the edge-on, but obscured the 'binucleate' character. Nearly-direct vision revealed the 'binucleate' character on occasion and even showed the elongation of the high surface brightness companion at a few instants, but made the edge-on look shorter"

And as for NGC 6928, I noticed that it gets thicker in the region south of the star that flanks it.

This is with an 18" Obsession and a 10mm Delos EP.

Clear Skies!
Akarsh