June 2016 - Galaxy of the Month
NGC 6702 in Lyra
This interactive image of the NGC 6702 and NGC 6703 was provided by the Digitised Sky Survey using Aladin Sky Atlas. You can download a finder chart for these galaxies.
Summer nights in northern climes are not very good for any form of deep sky observing, let alone hunting galaxies and I had debated whether or not to have a GOM for June and July. However just to keep the sequence going for June I decided to add a couple of galaxies in Lyra to the list, NGC 6703 and 6702.
Although NED suggests that they are a physical pair the fact that the recession velocity for NGC 6702 is 4727 km/s and NCG 6703 has a recession velocity is 2365 km/s suggests that they are not in fact physically associated but may be separated by 30 Mpc.
Visually they are challenging enough objects that they were not discovered by either of the Herschel’s but had to wait until 1863 when they were found by d’Arrest using an 11” refractor.
NGC 6702 is classified as an elliptical galaxy (E3) whilst 6703 is a lenticular (S0/E). Images from the DSS show that NGC 6703 appears to have a bright core and then a much fainter halo whilst NGC 6702 appears to be elongated and with a much fainter extension. For owners of larger telescopes there is also an interesting edge on galaxy UGC 11357 close by as well, although this is very much fainter.
NGC 6702 shows the attributes of a LINER galaxy, one that shows emission lines and probably a sign of a mild AGN or enhanced star formation. NGC 6703 is about 26 Mpc from us whilst NGC 6702 would appear to be nearer 60 Mpc distant. It is possible that NGC 6702 may have undergone a merger on a relatively recent (2 Gyr) timescale from both the colour and the fact it has two distinct globular cluster systems. This would also be borne out by the fact that deep images show a dust lane near the centre of the galaxy.
NGC 6702 was also the source of a recent supernova 2002cs which was discovered by the amateur astronomer Mark Armstrong and appeared to be a Type Ia, typical for an elliptical galaxy. Both of these galaxies may be seen with a 20cm telescope, although NGC 6702 will be tough to find and will require averted vison at the very least.
Owen Brazell - Galaxy Section Director
