Galaxy of the Month in Lyra

• 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

• June 2015 - Galaxy of the Month

  NGC 6745 in Lyra

  This interactive image of NGC 6745 was provided by the Sloan Digital Sky Survey using Aladin Sky Atlas. This Megastar 5 finder chart should help you locate this galaxy, and a few others too.

  Surprisingly a galaxy that William Herschel missed, NGC 6745 was discovered in 1875 by Edouard Stephan using a 31" silver on glass reflector.

  He described it as vF, IE, ns. This does not suggest a promising target but in fact it can be seen with much smaller telescopes. Made famous by the Hubble image it was described as a bird eating its prey.

  There are two galaxies catalogued here in NGC 6745A and B. The larger galaxy NGC 6745A has suffered a direct collision with the smaller galaxy NGC 6745B which is now leaving the scene rather than just gravitationally interacting with it. A galactic hit and run ☺. This has stirred up a large amount of star formation in NGC 6745 but little in the other.

  Many sources suggest there are three galaxies at this location but I think there are only two currently with the large area of enhanced star formation from the collision being listed as the third object. It maybe that in a 100 million years or so that this may be classified as a triple system because the large area of star formation does appear to be moving away from the larger galaxy and could then be classified as a compact dwarf galaxy. This large area of young massive star clusters may have been formed from a large knot of gas that was pulled from NGC 6745.

  The pair is estimated to be about 206 million light years (63.5 Mpc) away. The blue starburst may be only 10 million years old or so. The total collision time is likely to be of the order of hundreds of millions of years. Although the galaxies collided directly it is unlikely that any stars directly collided Instead what we see is that gas and dust that had been stirred up by the collision is now forming a new generation of stars in a form of starburst. The line of blue white stars traces the path of the smaller galaxy through the larger.

  NGC 6745 used to be a spiral galaxy but the collision has severely distorted its form. I find it interesting that this pair did not make Halton Arp’s famous catalogue of peculiar galaxies. There are observations of the galaxy on the Deep Sky Forum.

  Interestingly there is a small chain of MCG galaxies about 20 arcminutes north east of NGC 6745. See the accompany chart and image from the SDSS. The brightest two galaxies are at 15.5 magnitude and the third at about 17 magnitude so these will be targets for large telescopes I think.

  Owen Brazell - Galaxy Section Director