I finally had a string of good observing nights at the beginning of June and I spent parts of two of them with the 20” chasing down galaxies in the group around NGC 6107 in Corona Borealis. I had noticed in Uranometria that they would be well placed for observing, so I checked Wikisky. I noticed there were many more small, faint galaxies in the vicinity that should be in range of the 20”, so I printed out a chart of the SDSS III image, with a number of the galaxies marked to see what I could detect.
“Be vewy qwiet, I am hunting faint gawaxies!”
Per Steve Gottlieb’s NGC site there are seven NGC’s associated with the group, all discovered by Édouard Stephan in 1880 using a 31” reflector. This galaxy cluster is also catalogued as Zwicky 1615.8+3505 and per a 1995 Japanese study is a “dynamically young poor cluster.” However, it has very little star formation. Unfortunately, the study only examined brighter members between 12.9 and 15.1 V magnitude. Most are spirals with a few ellipticals. My estimate of the distance of the group is roughly 430 million light years based on an average velocity of about 9,000 km/sec.
A remark in the study notes that Ulrich in 1978 suggested that the cluster may in fact be two subclusters in the same line of sight based on velocity distribution. I have the same impression from examining images, observing members of both groups, and the reported members. There is a lot more here than the earlier surveys reported.
NGC 6107 is right next to a bright field star so it appears smaller and dimmer than it really is. The northern and more centrally located NGC 6109 appears the most luminous at 12.9 V mag. However, in the SDSS III images 6107 appears to be physically larger and perhaps brighter.
The somewhat dispersed NGC members of the cluster are NGC 6105, 6107, 6108, 6109, 6110, 6112, 6114, 6116. They range in velocity from 8,600+ to ~9,300 km/sec. Two other more widely separated NGC’s have been proposed as members, NGC 6104 at 8,382 km/sec and NGC 6097 at 9,963 km/sec. Sprinkled about, within and around the main group are at least another 30 galaxies which I observed, mostly quite small and faint. Others remain in the field which I either did not target or were too faint for conditions. Of the thirty I observed only one is clearly not a member, PGC 57717, a large elliptical galaxy, about 15.6 g mag, at about 1 billion light years. The other 29 galaxies had velocities ranging from ~8,100 to 10,600 km/sec and g magnitudes from 15.1 to 17.4.
It would be interesting to see all of the galaxies brighter than ~18.0 g mag in the vicinity plotted in 3D. I suspect this might reveal a vague/irregular double lobe shape facing us on end.
(Edit: To fix paragraph formatting that didn't come through.)
Edited by Redbetter, 13 June 2019 - 11:34 PM.