NGC 3090 in Sextants
March 2018 - Galaxy of the Month
There have been some comments that recent GOM’s have been too easy as some of the galaxies were bright enough to have been plotted on the Pocket Sky Atlas (PSA) 😊 This month then we go to Sextans and the small galaxy group around NGC 3090.
The six NGC galaxies in this group were discovered on the 22nd Jan 1865 by Albert Marth using William Lassell’s 48” telescope in Malta. This was one of the last of the large speculum metal mirror telescopes. For further information on Marth’s work with this telescope and his catalogue of nebulae see Alan Dowdell’s article in the Webb QJ 100, or of course Wolfgang’s monumental work on Observing and Cataloguing Nebulae and Clusters.
The group consists of NGC 3083, 3086, 3090, 3092, 3093 and 3101. It does appear to be a physical grouping and has the galaxy cluster catalogue number WBL 248. The galaxy CGCG 8-18 is also part of this group. As listed in the WBL the group contains 7 galaxies.
These galaxies will be much more challenging to find as most of them are in the 13-15th magnitude range. As expected this is too faint to appear in most of the classic references such as the Night Sky Observer's Guide (NSOG).
Most of the galaxies in the group are edge on (or close to it) spirals but NGC 3090 itself as an E4 galaxy, it is also classified as a cD (cluster dominant) galaxy. If the distances published are correct then the group is at a distance of about 108 Mpc, which would give NGC 3090 a diameter of maybe 200,000 lyrs, about twice the size of the Milky Way.
Recent observations of the group taking into account dwarf galaxies suggest that the total cluster membership maybe nearer 20 galaxies. Many of these surveys however where based on algorithms to find cluster members and hence they are based on statistical surveys, the galaxy group in this case was MZ 03587 from a study done by the 2DFGRS galaxy survey. The group is also known as MKW 1 from a survey of cD galaxies done by Morgan, Keenan and White in 1975. If these studies are right then NGC 3090 is a true poor cluster and it may well be a fossil cluster, the end of the merging process of a group of galaxies.
Observations of the group may well be hampered by the two 10th mag stars involved with it. It will be interesting to see what size aperture is required to pick up all the NGC galaxies in the group.
Owen Brazell - Galaxy Section Director