NGC 3395 in Leo Minor

March 2023 - Galaxy of the Month

This month’s challenge is the galaxy pair in Leo Minor, NGC 3395 and NGC 3396, also known as Arp 270 and VV 246. The pair were discovered by William Herschel in December 1785. They are part of the NGC 3430 group, also known as LGG 218, which lists about 15 members lying at about 30 Mpc.

There are a number of other fainter galaxies in the field that were discovered by Stephan Javelle using a 76cm refractor, these being IC 2604, IC 2605, IC 2608 and IC 2612, although IC 2605 is just a bright spot in NGC 3395. IC 2612 is not associated with the group being a background galaxy.

UV observations of the group with the GALEX satellite show that large amounts of star formation appear to be going on in both NGC 3395 and 3396 along with the nearby galaxy NGC 3430, in fact NGC 3395 is classified as having a starburst nucleus.

The galaxies appear to be in the early stages of an interaction and are still separate, although at the calculated distance they would only be about 12 kpc apart, about a third of the distance between our Milky Way and the LMC. The galaxies appear to be overlapping and a bridge and tidal tails have been detected in the pair. The suggestion is that they have already had one close pass and the second will be due in 10 million years or so.

NGC 3395 has been classified as a distorted spiral and NGC 3396 as Im (a Magellanic style irregular), although with its bright line spectrum it has also been classified as a Wolf-Rayet galaxy because of all the massive star formation going on. NGC 3395 was one of the galaxies that the Rosse team at thought to be spirals after observations using the 72” at Birr.

Perhaps surprisingly I can’t find an HST image of the pair. One assumes because they have strong infra-red emission, probably caused by the strong star formation, that at some point the JWST will have a look.

The NGC 3395/3396 pair is one of the best and brightest of the galaxies in the Arp atlas. NGC 3395 makes the Astronomical League’s H400 list, although the brighter of the pair, NGC 3396 does not.

The galaxies are a close pair so using high power to separate them will work well. A medium power field judiciously placed will also include the low surface brightness galaxy NGC 3430 in the same field of view as the others. This pair should be considerably easier to see than the last two GOM’s.

There are many reports of people seeing them with a 25cm (10”) scope which suggest both galaxies are easy to see. The Night Sky Observer's Guide Vol. 2 suggests that at a medium power in 40-45cm (16-18”) scopes the pair is well seen and looks like a butterfly. The nucleus of NGC 3395 is almost stellar. The pair have an entry in OOTW in the DSF forum.

Owen Brazell - Galaxy Section Director

NGC 3430 in Leo Minor

January 2018 - Galaxy of the Month

For the first time in the GOM column we head into the constellation of Leo Minor and the nice group of galaxies around NGC 3430. All of the NGC galaxies in the group (NGC 3430, NGC 3425, NGC 3413, NGC 3395 and NGC 3396) were discovered by William Herschel in 1785.

The area is also littered with IC objects that were discovered by Bigourdan, although almost none of them are real, he was obviously having a bad night!

The group is listed as LGG 218 which apparently contains 10 galaxies including the main core around NGC 3430. However, it appears from its radial velocity that NGC 3413 is not part of the physical group, just a line of sight addition.

NGC 3395/6 are also known as Arp 270 and it maybe that Bigourdan saw a knot in the spiral arm of NGC 3395 part of this galaxy pair when he reported the object that became IC 2605. This pair were also noted in the VV catalogue of interacting systems that preceded Arp’s work. Arp 270 has also featured in the DSF OOTW column.

NGC 3430 itself is classified as a Wolf-Rayet (WR) galaxy, a relatively rare type of galaxy undergoing a strong burst of star formation and showing emission lines in its spectra. The WR phenomena in galaxies are normally triggered by interactions between galaxies and it is thought that NGC 3430 is interacting with NGC 3424 and indeed deep images and radio observations suggest that both NGC 3430 and NGC 3424 do show signs of tidal interaction.

NGC 3430 was also one of the galaxies that the Rosse team at Birr thought were spiral in nature, and it appears that they were correct in that call in this case. With the exception of NGC 3413 which is a lenticular galaxy all of the others in the field are spiral in nature.

The group is probably of the order of 90 million light years away, although I have seen distances as close as 76 million light years quoted.

Arp 270 is a merging galaxy system, probably in the early stages of the merger and there is a lot of star formation going on. There is a sixth NGC galaxy which is also part of the LGG 218 group in NGC 3442 but this is almost a degree north of the main core around NGC 3430.

The main galaxies are bright enough to be seen in medium aperture telescopes, of the order of 30cm, and the group around NGC 3430 is a nice triple system when using high power. All five galaxies may appear on the same field when using a modern hyperwide eyepiece with a medium power of say 180x.

Also in the same area is IC 2604, a 14th magnitude galaxy that was discovered by Javelle in 1896. It is also thought to be part of the group and will be a challenge for owners of larger telescopes. So overall an interesting group which appears to have two independent interaction events going on in it.

Owen Brazell - Galaxy Section Director