Galaxy of the Month in Crater

• NGC 3637 in Crater

  April 2021 - Galaxy of the Month

  For this month’s challenge we dip down into the constellation of Crater. Unfortunately, this means our targets will not rise that high as seen from the southern UK.

  NGC 3636 and 3637 were both discovered by William Herschel in 1786 and then independently by Andrew Common in 1880. Although Common is perhaps best known now for his photographic work, the NGC objects he discovered were all found visually using his 36” reflector from Ealing near London. This is the same telescope that went on to become the Crossley reflector at Lick.

  The two galaxies appear to form a physical but non-interacting pair. NGC 3636 is classified as an E0 and NGC 3637 as a lenticular but of a complex form with a strong bar and a ring around it. This leads to the rather complex morphological classification of (R)SB0(r). The pair are associated with a group of galaxies associated with NGC 3672 known as LGG 235, which contains only these three galaxies. The group is fairly widely spread on the sky with NGC 3672 being over a degree away from the other two. As perhaps expected from their classifications these galaxies consist of old stars with no signs of star formation going on. Perhaps surprisingly not much research has been done on this pair and most of what has been done has been on the structure of NGC 3637. The group lies at a distance of perhaps 21Mpc from us. The pair merits a couple of pages in the Annals of the Deep Sky Volume 7.

  Observationally this group will be a challenge not only because of the low altitude at which it culminates but also because of the presence of the presence of the bright 7th magnitude star HD 98591 between them. Both galaxies have bright cores and it may well be this is all that can be seen of them.

  I note that in the glare of the bright star there is another galaxy that does not appear in any of the other main visual catalogues, probably because on the plates they were compiled from it was overwhelmed by the glare from the star. The galaxy is only listed in surveys in the IR from the WISE satellite and also in the UV from GALEX. It will be interesting to see if it is visible in larger telescopes but I suspect the glare of the star will be too much.

  My suspicion is that high power is going to be needed to work on this group so the unusual combination, for the UK anyway, of a clear transparent night with good seeing maybe required for this pair. The pair does make the Night Sky Observer's Guide (NSOG) Vol.2 which suggests they are a target for 12/14” scopes. Both galaxies make the Astronomical Leagues Herschel II list.

  Owen Brazell - Galaxy Section Director

• 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