ACO 779 in Lynx
February 2024 - Galaxy of the Month
This month’s challenge is the galaxy cluster ACO 779 in Lynx. Containing at least 7 galaxies found in the NGC this should be a decent target for medium to large scopes.
The BCG (Brightest Cluster galaxy) is the giant elliptical galaxy NGC 2832, classified as a cD galaxy, also classified as E2, NGC 2832 itself was discovered by William Herschel in 1785. Most of the rest of the group appear to have been discovered by Stoney using Lord Rosse’s 72” at Birr. The Lord Rosse team found 12 galaxies in this area.
The group seems to contain mostly spirals and lenticulars with an interacting group from the extended VV catalogue in the centre. The same pair is also known as Arp 315, although this may apply to the three galaxies NGC 2830, NGC 2831 and NGC 2832. The galaxies in Arp 315 do not appear to be interacting.
ACO 779 is quite a small cluster with maybe 83 members and lies maybe 300 million light-years away. The Abell richness class however is 0 which suggests that there may be only 30 galaxies actually in the group. It covers roughly 50 arcminutes on the sky, although deep observations suggest it is more like 90 arcminutes, and lies roughly 40 arcminutes south of Alpha Lyn so it should not be that hard to find.
The group also contains a number of blue compact dwarf galaxies which appear to be forming stars at this time. There are some suggestions given its distance and location that ACO 779 may be part of the extended substructure of the Coma galaxy filament. There does appear to be some confusion regarding the NGC numbers in the group. NGC 2832 is certain but the others have been shuffled around a bit over time.
There is a decent amateur image of the group on Bernhard Hubl's website. I think observing this cluster will be best done with a chart as some of the fainter galaxies may look stellar. There is an interesting drawing available on X (Twitter) was made with a relatively small telescope, although this observers drawings appear to have some question marks about them.
A chart for the cluster can be found in Alvin Huey’s guide to observing Abell Clusters at which can be downloaded from his website. Steve Gottlieb suggests that NGC 2832 is visible in 13” but the others require a larger telescope, which given he usually observes in the high mountains suggests the group may require 45-50 cm to pick up more than the main galaxy. The Night Sky Observer's Guide (NSOG) Vol.1 suggests that a 20cm may show NGC 2832 and 30-35cm will show one of the others but 40-45cm are needed to show more galaxies in the group. The group is included in the Astronomical League’s groups and galaxy clusters program as well as in the SAC list of galaxy clusters.
Owen Brazell - Galaxy Section Director
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Galaxy of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.