NGC 584 in Cetus

November 2018 - Galaxy of the Month

First discovered in 1785 by William Herschel, NGC 584 is an elliptical galaxy with a morphological classification of E4 in Cetus. It is accompanied by the spiral galaxy NGC 586, also discovered on the same night by William Herschel.

NGC 584 is also noted as IC 1712 as Barnard found it whilst observing the comet C/1888 R1 – one of his discoveries. He sent a note to Dreyer about it but then realised that it was NGC 584, which he neglected to tell Dreyer so the observation was added in as IC 1712.

Although NGC 584 is classified as an elliptical there is growing evidence that it may in fact be a lenticular, and in fact in the de Vaucouleurs Atlas of Galaxies (CUP) it is given as an example of an SA0-.

NGC 584 is part of a small group of about 8 galaxies catalogued as LGG 27 which also includes the galaxies NGC 586, NGC 596, NGC 600, NGC 615 and NGC 636. Most of the galaxies in this group are ellipticals or lenticulars. It is worth noting that other resources put the number of galaxies in the group as high as 11. William Herschel has the honour of discovering all the NGC galaxies in the group.

The group is only about 20 Mpc away from us and is spread quite widely across the sky with about 2 degrees separating NGC 584 and NGC 636. SIMBAD suggests NGC 584 is part of a pair of galaxies (one assumes with NGC 586). NED does not have this so I guess more work to be done, although NED does reference a paper which does suggest that NGC 586 is the companion. The pair are not obviously interacting. The NGC 584 group is also part of the Cetus II cloud.

Observationally, given that William Herschel found all these galaxies, they should not be that much of a challenge to find. NGC 584 itself is part of the Herschel 400 listing and O’Meara also has it has it as number 6 in his book of Hidden Treasures.

NGC 584, NGC 586, and NGC 596 should all fit in the same field of view of a modern hyperwide eyepiece at a medium power (say 160x). O’Meara suggests they are all visible in a 4” telescope, if you are at 7000’ up a mountain and in pristine skies. O’Meara also carries on the fashion of naming everything by calling NGC 584 the little spindle galaxy. Experienced galaxy observer Mark Stuart finds it only just visible with his 10” SkyWatcher from moderate skies in the UK.

For those who do not feel that NGC 584 is enough of a challenge then there are a number of other fainter galaxies in the area that will be a challenge for larger telescopes, including IC 127 and LEDA 1028168.

Owen Brazell - Galaxy Section Director

ACO 2197 Group in Hercules

May 2018 - Galaxy of the Month

When we think of galaxy clusters in Hercules people tend to focus on ACO 2151, perhaps better known as the Hercules cluster, and ignore the other two bright(ish) galaxy clusters ACO 2199 and ACO 2197. I have covered ACO 2199 in a very early Galaxy of the Month (GOM) when focusing on its brightest member NGC 6166 so this time we will focus on ACO 2197.

ACO 2197 is part of the great wall of galaxy clusters which includes the Coma Cluster (ACO 1656), the Leo Cluster (ACO 1367) and the Hercules Cluster (ACO 2151). It lies only a short distance on the sky, about 1.5 degrees, from ACO 2199 and there is evidence of a filament of galaxies linking these two clusters with ACO 2151. ACO 2197 and 2199 along with 2151, 2152, 2147 and 2162 comprise the Hercules Supercluster.

Classified as a type III cluster on the Bautz-Morgan system ACO 2197 is a fairly irregular cluster containing 11 galaxies catalogued in the NGC so it should perhaps be better known.

It is covered (as expected) in the Webb Deep-Sky Society Observer's Handbook (WSDSOH) Volume 5 and also on Albert Highe’s web page which includes a detailed list of galaxies in the cluster as well as a finder chart for the central region.

The brightest three galaxies in the cluster, NGC 6160, NGC 6173 and NGC 6175 were all discovered by William Herschel in 1787. At the same time Herschel also found the fainter galaxies later catalogued as NGC 6146 and NGC 6150. John Herschel also discovered a number of nebulae in the cluster but as these are getting quite faint the other NGC objects were discovered by the Rosse team and Stephan. The Rosse team did discover two other galaxies in this field that did not make it into the NGC.

ACO 2197 is quite a large cluster in spatial terms covering almost 1.5 degrees in length on the sky. A brief observation of this cluster with Andrew Robertson’s 610mm reflector on a very transparent night suggests that a lot of galaxies may be visible in the field (we had been following galaxies in the ACO 2199 cluster for such a distance I was checking we had not run into the ACO 2197 cluster by mistake).

As with ACO 2199, ACO 2197 is dominated by a giant cD galaxy, in this case NGC 6173 along with two other giant ellipticals (NGC 6146 and NGC 6160). The cluster is thought to be at a distance of perhaps 126 Mpc. In terms of galaxy numbers ACO 2197 would appear to contain maybe 1500 galaxies or more, so it is quite a large cluster. It may also be interacting with ACO 2199 as the clusters may only be 9.2 Mpc apart in space.

The cluster appears to form an E-W alignment based on the bright galaxies NGC 6146, NGC 6160 and NGC 6173, a line almost a degree long. As such even with modern wide field eyepieces this cluster is going to take quite a few fields to study. It may however be more tractable than the galaxy fields of Coma. It does seem that in general the galaxies congregate at either end of the cluster around NGC 6146 and NGC 6173.

One of the more interesting galaxies in the group is the double galaxy NGC 6175, which appears to be a spiral and an elliptical almost on top of each other. The pair would appear to be included in the updated VV catalogue as VV 1816, although I am not sure of evidence of interaction. It would be an interesting challenge to see what aperture telescope is needed to split this pair. As always good seeing will be needed as well as good transparency I would think.

Having said all this it is probably worth noting that the Night Sky Observer's Guide (NSOG) regards the brighter galaxies in this group as challenges for 16/18” telescopes.

Owen Brazell - Galaxy Section Director