September 2017 - Galaxy of the Month

Hickson 88 in Aquarius

For September's GOM we stay in the constellation of Aquarius and look at the galaxy group Hickson (HCG) 88.

This group unfortunately does not rise that high from the UK, only getting above one airmass in September so it may be a challenging target. HCG 88 is however one of the brighter Hickson groups, although that is not saying much.

The main group consists of four spiral galaxies NGC 6975, NGC 6976, NGC 6977 and NGC 6978, although the classification of the edge on galaxy (NGC 6975) is uncertain, it could be a barred spiral.

NGC 6975 was discovered by Bigourdan in 1886 whilst NGC 6876, NGC 6877 and NGC 6878 were discovered by Marth in 1863 and 1864 using Lassells 48” speculum metal telescope from Malta. Bigourdan also thought he had discovered two other galaxies in the area, NGC 6973 and 6980, but these turned out just to be stars that looked nebulous in poor seeing, something we have all had issues with 😊

Strangely SkyTools plots NGC 6980 as a cluster in the same field. I am not sure where that information came from.

The group appears to be physically interacting with the usual signs of distortions and tidal tails visible in deep images.

In the original Hickson catalogue the group was identified as having four members but deep imaging has added two more to the group. Redshift surveys suggest that there may even be more members.

The group appears to have a very low velocity dispersion therefore any interactions cannot have been between the main members of the group as the crossing time is equal to the Hubble time (age of the Universe).

There does appear however to be some controversy about the interaction state of this group. It is probable that HCG 88 is incorporated in a loose group of galaxies (not unusual for Hickson groups). That group may be UGCl 458, a poorly studied galaxy cluster that was also noted by Zwicky in the CGCG. The group is at a distance of about 84 Mpc from the redshift data.

NGC 6978 itself is also classified as a LINER (a form of AGN).

The group is likely to be fairly challenging to observe and may need apertures of the order of 40cm or more to see much, except from very dark skies. I have seen observations however that suggest that the cores of the brighter galaxies may be seen with smaller instruments.

The main group appears as a nice triple but my suspicions are that from typical UK skies a 50cm telescope may be needed to see NGC 6975. It would seem that medium to high power are best to use when observing this object. As always try when the group is within a couple of hours of the meridian. There is also the 6th magnitude star 4 Aqr nearby so you will need to use reasonable magnification to keep that out of the field.

Owen Brazell - Galaxy Section Director