Hickson 62 in Virgo
May 2019 - Galaxy of the Month
With the skies nominally no longer dark from May onwards at my latitude to see any galaxies at all means I need to choose brighter targets. This month it is the bright galaxy group Hickson 62.
The main pair here are NGC 4776 and NGC 4778. Both of these galaxies were discovered by John Herschel in May 1836. The third NGC galaxy in the group, NGC 4761, was discovered by Tempel in 1882 along with the fourth galaxy NGC 4764.
By today’s standards Tempel was using a relatively small telescope in an 11” refractor but he was an exceptional observer and he described them as eF, so they are going to be a challenge to find.
As seems often in the NGC there is some confusion here about the designations and it appears that William Herschel did see the brighter galaxy pair but could not resolve them into two so he described it as a double nebula and it was entered into the NGC as NGC 4759. Continuing confusion comes with some of the professional databases like SIMBAD where NGC 4778 is equated with NGC 4761, which is the small galaxy NE of the double pair. Even the identity of NGC 4764 is uncertain because Tempel did not give Dreyer co-ordinates and they made up a position for it.
As such there is a wide variation in sky charting programs as to what galaxy is which. Skytools for instance appears to have a confused set of designations in both SkyTools 3 and 4. SkySafari 6 appears to fit the corrected NGC view of the group.
Views of the group are going to be complicated by the 8th magnitude star situated in the middle of the group of galaxies. The compact nature of the group also suggests that you are going to need to use high power on it to separate the a/b pair and probably to bring out the others.
Hickson 62 is interesting because it is one of the closest compact groups and by studying it in X-Rays with Chandra we can see a very hot (several million degree) intra cluster medium. This intra cluster medium is not smooth and appears to have several cavities in it which may be what is stopping a classical cooling flow happening. These cavities could be the remnants of radio lobes from the past when NGC 4778’s central engine was more active.
Currently NGC 4778 is classified as a low luminosity AGN. Observations of NGC 4778 also suggest that it may have had a recent merger with a much smaller galaxy. NGC 4778 is classified as an S0, a lenticular, as indeed is NGC 4776. The other two galaxies in the group are also either elliptical or lenticulars. The group appears to be at a distance of about 60Mpc.
Observationally NGC 4778 appears in Luginbuhl and Skiff (L&S) under the NGC 4759 moniker where they suggest that it may be visible as a faint patch with a 15-cm. I would think that from UK skies at least 20-cm would be needed to repeat that observation. Interestingly the pair do not appear in the Night Sky Observer's Guide (NSOG) under any designation. The group is part of the Astronomical League Galaxy Groups and Clusters list.
Owen Brazell - Galaxy Section Director