NGC 3637 in Crater

April 2021 - Galaxy of the Month

This interactive image of the NGC 3637 and was provided by the Sloan Digital Sky Survey using Aladin Sky Atlas. We also have a finder chart should help you locate these galaxies, as will this link for NGC 3637 on the Stellarium Web planetarium.

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