NGC 1684 in Orion
December 2017 - Galaxy of the Month
The small group of galaxies around NGC 1684 in Orion, including NGC 1682, NGC 1683 and NGC 1685 will provide a challenge for most observers.
The brightest two, NGC 1682 and NGC 1684 were discovered by William Herschel in 1786 but it took the power of the 72” at Birr for Stoney to discover NGC 1683 and NGC 1685 in 1850.
The group seems to contain one large elliptical in NGC 1684 along with three lenticular, or possibly spiral galaxies. NGC 1685 seems to be clearly a barred spiral. They seem to be forming a physical grouping and, if the distances are correct, it is about 204 million light years from us.
The grouping is fairly tight as seen on the sky and should fit in a medium – to high power eyepiece field using one of the modern hyperwide eyepieces. Unfortunately, the group does not rise that high from the UK, culminating at about 35 degrees, so a good dark southern horizon will be needed. Sue French in her Deep Sky Wonders column from Feb 2005 finds the two main galaxies visible in her 4.7” telescope relatively easily but I suspect most people will struggle with 20-cm.
On a night with poor transparency I found them relatively easily with my 37-cm telescope, although NGC 1682 was the more difficult of the pair. I thought I saw NGC 1685 as well.
NGC 1684 has a classification of E2 Pec so it is going to be a pretty round and featureless galaxy. NED has it classified as a cD Pec galaxy, which suggests that it is the dominant galaxy in a cluster and also has some AGN like properties, i.e. it contains a massive central black hole.
The group, if it is a physical one, has been defined as a compact grouping. Surprisingly, if it is a group, then it does not seem to appear in either the WBL or LGG catalogues.
In UV images from GALEX it is NGC 1685 and MCG -1-13-33 which show up the strongest as these galaxies obviously have a lot of active star formation whereas NGC 1682 and NGC 1684 do not register at all.
If these NGC galaxies are too easy there are also a number of MCG galaxies in the same area, including MCG -1-13-22, which appears to be a bright(ish) edge on galaxy and MCG -1-13-33. At mag 15.1 MCG -1-13-22 may be easier to see than NGC 1683 at 15.6.
It would appear that all of these are visible in 45-cm, albeit with averted vision, on a good night. It is worth bearing in mind that most of the magnitudes quoted would be B or photographic ones so they galaxies may be up to a magnitude brighter when seen visually, as indeed some software suggests.
Owen Brazell - Galaxy Section Director