Hickson 40 in Hydra

February 2023 - Galaxy of the Month

This interactive image of the Hickson 40 was provided by the Pan-STARRS1 Surveys using Aladin Sky Atlas. We also have a finder chart that should help you locate these galaxies, as will this link for Hickson 40 on the Stellarium Web planetarium.

After last months really rather hard challenge we have something slightly easier this month in the galaxy group Hickson 40, also known as Arp 321 and VV 116, in Hydra.

Although this may be challenging because of its altitude from northern climes it is a really nice tight group. Consisting of three spirals, an elliptical and a lenticular galaxy this is one of the tightest groups in the Hickson catalogue. All the galaxies appear to contain compact sources, suggested to be supermassive black holes, at their centres. The size of the group is so small that all five galaxies could fit in a volume less than twice the size of our Milky Way galaxy.

Although such compact groupings are regularly found in large galaxy clusters Hickson 40 appears to be a rather isolated field group. There actually appear to be 7 galaxies in the group but only 5 are easily seen. The group is located perhaps 300 million light-years away and is in the process of undergoing a merger to form one large elliptical galaxy. This may take place in a billion years or so.

Hubble imaged these galaxies as part of its 32 birthday study. The group was classified by Arp as part of his class of galaxy groups.

This group is going to be challenging for observers as none of the galaxies are bright enough to have an NGC or IC designation and the low altitude as seen from the UK, although it does crawl above the 30 degree line, will not help either.

The brightest galaxy of the group is MCG 1-25-9 at 13.8 B so it should be visible in 30-40cm scopes from a dark sky. The rest of the group is somewhat fainter. The group is so tight that you should try using the highest power your telescope and the conditions will allow as this is also going to be the best when trying to tease out the details of the fainter members of the group.

Unfortunately, despite being a Hickson group it does not appear in the Night Sky Observer's Guide Vol. 2. There is some information on the group on Reiner Vogel’s Hickson page.

Owen Brazell - Galaxy Section Director