April 2016 - Galaxy of the Month

NGC 3865 and NGC 3866 in Crater

This interactive image of the NGC 3865 was provided by the Digitised Sky Survey using Aladin Sky Atlas. You can download a finder chart for the area. There's a SkyTools chart for the position of NGC 3854. You're advised to read on to find out why that might be important!

The constellation of Crater is often ignored from mid-northern latitude because it never rises very high above the murk and is not a distinctive constellation like Corvus. There are however a number of interesting galaxies in this area.

The galaxy of the month this month is the pairing known as NGC 3865 and 3866. However as always in the NGC if only it was that easy as the pairing are also known as NGC 3854 and 3858 and the literature seems confused as to what designation to use for them. So for instance Megastar and NSOG use NGC 3865 and 3866 as the primary designation whilst SkyTools for instance uses NGC 3854 and 3858.

The galaxies were originally found visually in 1880 by Andrew Common using a 36” reflector and Dreyer uses the 3865 and 3866 numbers for that discovery. Common’s positions however were not very accurate so they were rediscovered in 1886 by Leavenworth using a 26” refractor. His positions were notoriously bad (at least in RA) so Dreyer added these as new objects in the NGC with the numbers 3854 and 3858.

The whole sorry story is described by Harold Corwin in his NGC notes (http://haroldcorwin.net/ngcic/) . The problem comes in deciding what NGC number to call the objects because as we have seen they are already mixed up. In theory as Common found them first they should be called by his numbers.

Interestingly there is some suggestion that this pair is part of a loose group of 18 galaxies, however this comes from a statistical study of galaxies from the 2MASS survey so the reality of this group may be questionable. The group is not listed in any of the other optical galaxy group catalogues.

NGC 3854 is a barred spiral but does have odd looking spiral arms, perhaps as a result of some form of interaction. It is also very prominent in both the 2MASS near IR images and the GALEX UV images. Hubble images show a very bright core with complex dust clouds.

NGC 3858 also looks like it may be distorted but the image is complicated by a second bright source near the nucleus which may be a superimposed star or perhaps part of the galaxy. Unfortunately, there are no detailed images of the galaxy to resolve this issue. NGC 3858 is also classified a type 2 Seyfert. It appears to be part of the group with 3854.

L&S suggest that 3854 (there listed as 3865) is barely visible in 15cm but visible in 30cm. They also suggest that 3866 is just visible in 30cm. Night Sky Observer's Guide (NSOG) suggest both are targets for 40-45cm telescopes and 3866 is tough. These observations will be from much further south than the UK so it will be interesting to see what can be seen from here.

Owen Brazell - Galaxy Section Director