NGC 6109 in Corona Borealis

April 2019 - Galaxy of the Month

The small group of galaxies around NGC 6109 were discovered by Edouard Stephan over several nights in July 1880 using the Foucault designed 31” silver on glass reflector at Marseille, they must have dark nights in the summer there and good weather as the telescope seems to have sat outside rather than in any kind of enclosure.

The group of galaxies including NGC 6109 is also included in the WBL catalogue as number 612. It lists 10 galaxies including NGC 6105, NGC 6107, NGC 6108, NGC 6109, NGC 6110, NGC 6112, NGC 6114 and NGC 6116 as being part of the group. The group appears to be known after its brightest galaxy, NGC 6107. It is considered to be a dynamically young poor cluster.

Zwicky also noted this group as a cluster with the designation Zwicky 1615.8+3505. The Zwicky cluster listing also included NGC 6104 as part of the group but it is quite a long way out from the main group and the WBL listing does not include it as part of the group. It is also odd in that only NGC 6104 shows any signs of star formation.

The group forms a nice line of galaxies but as Stephan found them faint these may turn out to be a bit of a challenge.

NGC 6109 is both a radio source and a point X-Ray source and is currently classified as a LINER type AGN with an unusual doughnut shaped jet. Oddly for a galaxy of this type it is suspected to be an S0, lenticular. It also shows a typical head-tail radio structure, which is unusual as these galaxies are normally only found in rich clusters of galaxies, although this interpretation has been challenged and various models have been put forward to explain this feature, including a deflected jet. NGC 6109 was also home to supernova 2010an.

With the exception of NGC 6107, which is classified as an elliptical (E3?), most of the galaxies in the group are suspected to be spirals. NGC 6107 is also a radio and X-Ray source. The group is suspected to be about 400 million light years away.

There are also a large number of much fainter galaxies in the field which may be visible in very large amateur telescopes. Steve Gottlieb in his NGC notes describes all of the galaxies in the group as faint with an 18” from good skies so they are likely to challenge 20” and above from typical UK skies. Both NGC 6107 and NGC 6109 are included in the Night Sky Observer's Guide (NSOG) Volume 2 as targets for 16-18 inch telescopes.

Owen Brazell - Galaxy Section Director