NGC7385 in Pegasus
October 2018 - Galaxy of the Month
Staying in Pegasus for another month, we look at the small group of galaxies surrounding NGC 7385.
The New General Catalogue of Nebulae and Clusters of Stars (NGC) lists eight galaxies in this area, with the group also known as WBL 688. The two brightest galaxies, NGC 7385 and NGC 7386, were discovered by William Herschel in 1784. The rest may have been discovered by the Rosse team in 1850. However in the accompanying chart some of the numbers are incorrectly attached to galaxies when it would seem that the Rosse team actually only saw stars. So the numbers for NGC 7384 and NGC 7388 appear to have been randomly assigned to galaxies that were not actually seen by the Birr observers, but by later researchers.
Strangely, most modern references point out that these observations were just stars, so I am not sure who assigned the numbers to the galaxies. The compilers of the Revised New General Catalogue (RNGC) in this case do not seem to be guilty as they also refer to them as stellar. It seems that it was the compilers of the original Principal Galaxy Catalogue (PGC) in 1989 who were responsible for assigning these faint galaxies to the NGC numbers.
The WBL 688 group only lists 6 galaxies as physically associated in this area: NGC 7385, NGC 7386, NGC 7383, NGC 7389, NGC 7387 and NGC 7390. The group would appear to be at a distance of about 100 Mpc and to consist primarily of elliptical and lenticular galaxies. I wonder if this is the core of a fossil cluster. NASA/IPAC Extragalactic Database (NED) also lists the brighter galaxies as part of ACO 2506, which I struggle to understand, in fact discussions with Harold Corwin indicate this is an error.
When you look at deep images of NGC 7389 it does look just like a barred spiral galaxy, just without any obvious signs of new stars. This is in essence what lenticular galaxies are. They have the light distribution of spiral galaxies but no gas to make new stars. In this case NGC 7389 would be classified as an SB0 galaxy. NGC 7383 also looks very similar and is also classified SB0, although it does not show such a well developed bar. These shapes do not show up well on the DSS images but do on the SDSS and PanSTARRS images.
NGC 7385 appears to have radio jets, and in one of those an HII region has been detected at radio wavelengths which may be caused by the interaction between the jet and the ISM: one would not expect to see HII regions in a normal elliptical galaxy. Strangely SIMBAD refers to NGC 7385 as a quasar and, although it is certainly some form of AGN, most references suggest it is a low power one. NGC 7385 was home to SN 2005er.
The group is fairly tight and most of the galaxies will fit in the field of a modern high power hyperwide field (100 degree) eyepiece. It would appear that a 16” (40-cm) telescope will show five of these galaxies in this field, so I guess that the challenge for owners of larger telescopes to see more of the cluster.
If you wish to know more about this group then Mark Bratton published an excellent article on observing it in DSO 168, and for further reading on the observations by the Birr team see Wolfgang’s excellent book on Observing and Cataloguing Nebulae and Star Clusters.
Owen Brazell - Galaxy Section Director