December 2015 - Galaxy of the Month

NGC 691 Group in Aries

This interactive image of the NGC 691 group was provided by the Sloan Digital Sky Survey using Aladin Sky Atlas.

The NGC 691 group is a collection of 9 galaxies consisting of NGC 678, 680, 691, 694, 695 and NGC 695 along with IC 1730 and IC 167.

The group is spread over quite a wide area with almost a degree spanning NGC 691 and 695. The core of the group is centred around NGC 691, 678, 680 and NGC 694. NGC 678 and 680 were discovered in William Herschel in 1784 but it was two years later in 1786 before he found 691. Heinrich d’Arrest found NGC 694 in 1861 as it is much fainter than the others. Perhaps unusually for an IC object Bigourdan found IC 167 using a 12” refractor in 1889.

The whole group is spread around the bright double star 1 Ari. This may make it more difficult to see than it should be.

The group is about 122 million ly from us which is relatively close by and explains the large angular spread of the group on the sky.

NGC691 was home to SN 2005W, although it only reached magnitude 15.2.

Both NGC 691 and NGC 678 feature in the Webb Deep-Sky Society Observer's Handbook (WSDSOH) Volume 4 where both are described as brightish in a 16” telescope.

Night Sky Observer's Guide (NSOG) reports the two brighter members as easy in 22cm and the others as more challenging.

Interestingly most of the galaxies are spirals or lenticulars with only NGC 680 a confirmed elliptical, and a peculiar one at that as it shows signs of recent interactions or mergers with a number of shells. NGC 680 also appears to be interacting with NGC 678 as well.

There is an interesting amateur image of the whole group at Astrophotography by Alson Wong.

Split away from the main group on the other side of 1 Ari are NGC 697 and 695. NGC 697 is a nice bright spiral but NGC 695 is much fainter and although catalogued as a double galaxy its faintness suggests it might not actually be a member of the group. Its recession velocity from NED, if correct, is 3 times greater than the others in the group. It certainly appears in front of a field of much fainter galaxies. There also appears to be some confusion over the identity of NGC 697 with Harold Corwin suggesting it is actually NGC 674 and d’Arrest was confused and saw the same object twice.

The Megastar chart with this does not identify IC 1730, so there is a second more detailed chart from SkyTools that does.

Owen Brazell - Galaxy Section Director