Galaxies Section

Welcome to the Galaxies Section of the Webb Deep-Sky Society. Our role is to promote visual observation and scientific imaging of galaxies by amateur astronomers.

Galaxy of the Month

Below is our current Galaxy of the Month. These are galaxies that we feel are particular worthy of your precious observing time. We have an archive of these articles that stretch back to the beginning of 2011. You can browse them, or search for specific objects and constellations in the Galaxy of the Month archives.

Your observations (sketches, images or observing notes) of any of these objects are very welcome.

For those that use observation planning software

We have plans available for all the Galaxy of the Month pieces from 2011 onwards to make it easier to find the galaxies we have covered so far.

These will be updated as new GOM’s are added.

NGC 7436 in Pegasus

November 2019 - Galaxy of the Month

This interactive image of the NGC 7436 group was provided by the Sloan Digital Sky Survey using Aladin Sky Atlas. We also have a finder chart should help you locate these galaxies.

This small tight group of galaxies in Pegasus has an interesting discovery history. The brightest galaxy NGC 7436 was discovered by William Herschel in December 1784. However he was not able to resolve the other members of the group and it took Mitchell using the 72” at Birr to find NGC 7433 and NGC 7435 in 1855 and then Bigourdan added the last NGC member of the group (NGC 7431) in 1886.

The group are also listed as VV 84 in the Voronstov-Velyaminov catalogue of interacting galaxies where he included it in his group of triples with a tight pair, although the group did not get the attention of Arp.

Dreyer did note another galaxy in the group in 1875 but was not confident enough in his sighting as it was a bad night to list it. This may be the faint galaxy MCG +4-54-7.

Unfortunately there is some confusion about the NGC designations in this group with the nomenclatures NGC 7436A and B being used for the central galaxy and not necessarily for the same objects.

Mitchell and Dreyer did provide drawings of the field however which helps in trying to establish what galaxies they saw. The associated chart and image below will show how different galaxies are assigned different numbers.

Labelled image of the NGC 7436 galaxy group - Image provided by the Sloan Digital Sky Survey (SDSS)
A labelled image of the NGC 7436 galaxy group - Image provided by the Sloan Digital Sky Survey (SDSS)

The group also exists as part of WBL 692, which appears to consist of just the galaxies catalogued in the NGC. The group, also known as Holmberg 800, lies on the outskirts of the galaxy cluster Abell 2153 and is at about 340 million light-years distant from us.

NGC 7436 itself appears to be an elliptical galaxy with a large outer halo, suggestive of it snacking on other galaxies in the past. That fuzziness makes it difficult to decide if the little galaxy next to it lies in front or behind NGC 7436. NGC 7435 shows signs of an interaction with its distorted spiral arms although it is probably not with NGC 7436 as its recession velocity suggests that it lies behind it. It may be that NGC 7436 is only marginally part of the group of galaxies that we see. There are a number of other faint galaxies in the field including UGC 12274.

The compactness of the group suggests that to get much out of this will require a night of steady seeing and high power. Observations from Andrew Robertson using a 24” and Mike Wood using a 15” Obsession with an EAA system are included below. Also see Steve Gottlieb’s observations with a 48” dobsonian on his website.

Observational sketch of the NGC 7436 galaxy group by Andrew Robertson
An observational sketch of the NGC 7436 galaxy group and notes by Andrew Robertson with his 24" dobsonian (click on it for a larger version of the image).
EAA capture of the NGC 7436 galaxy group by Mike Wood
An EAA capture of the NGC 7436 galaxy group by Mike Wood using a 15” Obsession (click on it for a larger version of the image).

The group is also included in Alvin Huey’s selected small galaxy groups atlas and his VV atlas Part 1.

Owen Brazell - Galaxy Section Director