NGC 6661 in Hercules

August 2021 - Galaxy of the Month

This interactive image of the NGC 6661 and was provided by the Pan-STARRS1 Surveys using Aladin Sky Atlas. We also have a finder chart should help you locate these galaxies, as will this link for NGC 6661 on the Stellarium Web planetarium.

With the return of dark skies in August we can move to some more challenging targets for the GOM. This month I have chosen the faintish pair of galaxies in eastern Hercules, NGC 6661 and NGC 6658.

Both these galaxies were initially discovered by Albert Marth in 1864 using William Lassell’s 48 speculum metal telescope in Malta. However confusion was added to the area when Lewis Swift using his 16” refractor reported another nebula in the area in 1885 which was recorded as NGC 6660. It was later shown that this object was identical with Marth’s object NGC 6661 so it can have both NGC numbers. Edouard Stephan also independently rediscovered it in 1871. The correction was noted in the notes accompanying the IC1 in 1895 so it is a long-known issue.

Both of these galaxies are classified as S0-a, so they are lenticular galaxies. NGC 6658 appears to be more like an edge on spiral but shows no spiral arms or dust features. It does however appear to have quite a bright nucleus. NGC 6661 is in range of 25cm from the UK but shows only as a very faint object.

NGC 6658 and NGC 6661 are suggested to be a non-interacting pair according to the Second Reference Catalogue of Bright Galaxies (RC2). The distance to NGC 6661 is suggested to be about 42Mpc. There is some doubt however about them being a physical pair as NGC 6658 is listed as being part of the galaxy group LGG 421 and NGC 6661 is not. The other NGC galaxies associated with LGG 421 are NGC 6641 and NGC 6669. This would fit with their redshifts being vastly different with NGC 6661 being the further away (or moving much faster) so the pair appear to be just a line-of-sight effect. Interestingly though, as lenticular galaxies you would expect to see them as part of a group, or at the very least a fossil group if the understanding of how lenticular galaxies form is correct. NGC 6658 appears to be the brightest galaxy in LGG 421 but interestingly shows very little radio emission.

Perhaps not unsurprisingly the galaxy pair is not included in the Night Sky Observer's Guide (NSOG). As the pair is quite tight it should fit within a high-power eyepiece field of view, and indeed because of the faintness of the pair you are probably going to need to use medium to high power in order to boost the contrast enough to see the them. Although as noted a 25cm will just about pick up NGC 6661 I suspect that probably 40-50cm will be needed to see NGC 6658 given its discovery history, although again there are reports of it being seen on the edge with a 25cm telescope from UK skies.

Owen Brazell - Galaxy Section Director