NGC 7771 Trio in Pegasus

September 2022 - Galaxy of the Month

For the September GOM I have chosen the tight galaxy trio in Pegasus around NGC 7771.

There are three NGC galaxies in the trio, NGC 7769, NGC 7770 and NGC 7771. All three of the galaxies were discovered on the same night by William Herschel in September 1784. For EAA observers there are two much fainter galaxies in the same field. The three galaxies appear to form a physical group along with LEDA 214993, also known as NGC 7771a. They are also classified as Holmberg 820. They also make it into both the WBL (726) and LGG (483) galaxy group catalogues.

Both NGC 7770 and NGC 7771 appear to be slightly distorted from a gravitational interaction. All three appear to be spiral galaxies with all of them appearing to have a lot of star formation going on from the brightness in the UV images from GALEX. NGC 7771 is also classified as a luminous infrared galaxy (LIRG) starburst which also suggests that a lot of star formation going on is hidden by dust. It will be interesting to see if Webb images it as Hubble used its near infra-red camera to get an image of the centre of NGC 7771.

The distance to the group is thought to be about 60Mpc. The group appears to have been interacting for some time as there are numerous star streams in the group. Over cosmic time all three galaxies are likely to merge. It is not clear if NGC 7771a is part of the triplet as there is very little hard information on it. Unfortunately deep images of the group are complicated by the presence of dust (IFN) in our own galaxy in front of the group, although there is a fine amateur image of the group by Capella observatory showing some of the interaction details. Despite the group not make the Arp atlas it is added as VV 2003 in the revised Vorontsov-Velyaminov catalogue, although this may only apply to NGC 7770.

The Type II supernova SN2022mxv was discovered in NGC 7769 but only reached maybe 15th magnitude. NGC 7769 is also suspected to be a LINER, a weak form of AGN.

The Night Sky Observer's Guide (NSOG) Vol. 1 suggests that to see the whole group then you will require a telescope in the 40-45cm class and even then NGC 7770 will only appear as a faint smudge. The brightest galaxy in the Trio is actually NGC 7769 so that should be the easiest to find. As the group is so tight it will be worth using the highest power you can get away with to separate them and to increase the contrast to see NGC 7770.

The group also makes it into the Cambridge Photographic Atlas of Galaxies. They are also included in Alvin Huey’s guide to Galaxy Trios as well as Miles Paul’s Atlas of Galaxy Trios, downloadable from the Webb Society website. The group has been included in the Herschel 300 list, an extension to the better known H400 and H400 II list.

Owen Brazell - Galaxy Section Director