View Full Version : Ngc 6691

2011-Nov-11, 09:57 PM
NGC 6691 is a spiral galaxy located in Draco some 260 million light-years from us. Arp has a category in his atlas for 3 armed spirals. All three he included are really two armed spirals with a major spur off one to make the third arm. Thus, I include this one as falling into the same category. It has two main arms coming off the bar and a large spur overlaying one of those arms that first goes north on the western side of the galaxy they a second spur comes off it going northeast. Much smaller spurs are seeing coming off the other arm. The galaxy to the south is listed in a note at NED as being a companion. This field is so poorly studied it has no catalog entry in NED. All other galaxies in the image that NED identifies are from the 2MASS IR catalog which only picks up galaxies bright in 2 micron IR radiation. The companion apparently doesn't have enough.

In the cropped and enlarged image I've put NGC 6691 well off center to catch, in the opposite corner, two or maybe three galaxies. The two brightest are in the 2MASS catalog as 2MASX J18400484+5545142 and 2MASX J18400579+5545152. I can't tell for sure if the former is separate from the disk galaxy behind it or just a very bright core. I tend to think it is just another elliptical/S0 like the other one to the east and the disk object is a third galaxy whose core is mostly hidden by the 2MASS galaxy. NED is of no help here listing no size for either of the two galaxies.

14" LX200R @ f/10, L=4x10' RGB=2x10', STL-11000XM, Paramount ME


2011-Nov-21, 03:45 AM
Rick that is a really nice image, well done and great info as usual! I am curious as to how big it is compared to the MW? I tried Google, but could not find any additional info other than what you have already posted.

2011-Nov-21, 08:58 AM
All my full images are at 1" per pixel. That and the distance of 260 million light-years will give you its size with a bit of beginning trigonometry. I counted 95 pixels east to west. NED says its diameter is 1.6 minutes of arc. 95 pixels would be 1.58 minutes so we have a good agreement. Taking the tangent and multiplying by 260,000 gives 121,000 light-years. So it is a good match for our galaxy. Math profs will say the correct formula is to take the tangent of the radius rather than the diameter and calculate the radius then double to get the diameter. That is more accurate but for this small of an angle the difference is .00655 light years. I think that can be ignored considering the distance is likely only known to about +/- 20%.