Abell 1185 in Ursa Major

March 2022 - Galaxy of the Month

This month’s challenge is the often-overlooked galaxy cluster Abell 1185 in Ursa Major. Perhaps not unsurprisingly it was covered 40 years ago in the Webb Deep-Sky Society Observer's Handbook (WSDSOH) Volume 5.

The core of the group contains 6 galaxies listed in the NGC, with the brightest being NGC 3350. NGC 3350 was discovered by William Herschel in 1785. Herschel also found NGC 3552, along with an outlier to the main group in NGC 3527 at the same time. John Herschel found NGC 3554 along with 3561 whilst revisiting his father’s observations. Heinrich d’Arrest found NGC 3558 in 1866. The sixth is classified as NGC 3561A.

The cluster is thought to be about 400 million light-years from us and perhaps 1 million light-years in width. It does feature in a Hubble image at ESA/NASA website. Here you are of course just viewing the very centre of the cluster. There is also a wider field view of the cluster. Abell 1185 is thought to be a member of the Leo Supercluster and is the brightest member of that grouping. The suggestion is that the cluster contains at least 85 galaxies and has an Abell richness class of 1, i.e. not very rich 😊

The cluster seems to contain a large number of interacting galaxies with two appearing in the Vorontsov-Velyaminov (VV) catalogue with NGC 3561 being VV 237 and NGC 3530 being VV 1419. The cluster also contains Ambartsumian’s knot (a dwarf galaxy at the end of the banjo like tidal feature associated with NGC 3561A, also known as Arp 105). This feature is also known as the Guitar. NGC 3561 also contains a weak AGN of the LINER type. The group does seem to contain quite a few spirals and perhaps lenticulars which suggests it is quite a young cluster still coming together. There are also various sub clumps within the cluster which supports this theory. Abell 1185 also seems to be home to a large number of wandering globular star clusters that are not attached to any galaxy.

The cluster is a very compact one with all the major NGC galaxies fitting in the same field in a high power (345x) modern hyperwide eyepiece. This will probably be the best way to see them as the individual galaxies are not that bright, even NGC 3550 comes in around 14th magnitude photographic. The field also contains a number of other galaxies that have been catalogued in the MCG catalogue. Interestingly NGC 3550 appears to have three cores (or other galaxies superimposed on it). Are these visible? The other galaxy that Herschel found in this area, NGC 3527, which is also thought to be part of the group lies over 45’ away from NGC 3550. The group also makes the Astronomical Leagues Galaxy group and Cluster list. The faintness and tightness of the group means it may also be a suitable challenge for EAA observers. Andrew Robertson using his 24” managed to find 7 members of the cluster.

You may also need to look this group up as ACO 1185 which is now the preferred prefix for Abell galaxy clusters supplanting the older AGC description which is now used for a different catalogue.

Owen Brazell - Galaxy Section Director