NGC 750 in Triangulum

September 2019 - Galaxy of the Month

The interacting galaxy pair NGC 750/751, also known as Arp 166, was first discovered by William Herschel in 1784, although he only saw one galaxy here. It took Bindon Stoney using Lord Rosse’s 72” at Birr to see the fainter companion galaxy NGC 751.

The pair lie about 225 million light-years away in Triangulum and consists of a pair of elliptical galaxies. They may be only about 10kpc apart. The first interaction of this pair, which occurred about 100 million years ago, has drawn out a number of tidal tails. It is suggested that these two galaxies are still in the process of tidally interacting. Arp characterised them as galaxies with diffuse filaments.

NGC 750 also appears to be part of a group of galaxies known as the LGG 42, although, with the exception of NGC 761, none of the other galaxies in the field appear to be part of this group. The field consists of a nice chain of galaxies including NGC 736, NGC 740 and NGC 761.

There are a number of other NGC objects in this field but at least two of these, NGC 733 and NGC 760, along with NGC 737 are just stars. It is just possible that Stoney saw a faint galaxy near the star marked as NGC 733 so maybe it could be real but the historical evidence points to the star. Some software programs do however to assign the number NGC 733 to the galaxy rather than the star.

The lower group of galaxies around NGC 736 including NGC 738 and NGC 740 were discovered by Stoney in 1850, again using the 72”, as part of their survey of objects discovered by William Herschel. Herschel discovered NGC 736 in 1784. Stoney also discovered the northernmost galaxy of the chain, NGC 761 in 1850. It took Ralph Copeland in 1874 to find the last NGC galaxy in the group, NGC 739, also using the 72”.

Perhaps not surprisingly the NGC 750/751 pair also appears in the Vorontsov-Velyaminov (VV) catalogue of interacting galaxies as VV 189. What is interesting though is NGC 761 also appears in the VV catalogue as VV 425. NGC 761 is a nice spiral but there is also a fainter galaxy seen through the spiral arms. I suspect that this is a much more distant galaxy just seen in projection rather than an interacting system, however Vorontsov-Velyaminov added it as part of his M51 type systems. Interestingly NGC 736 also shows many shells which suggests signs of a recent interaction.

Given that only two of the galaxies in this field were seen by William Herschel I suspect that the rest of the group maybe a bit of a challenge to see and will require medium to high powers and probably at least 40-50cm aperture. The whole group will fit in a medium power field, say 200x, using a modern hyperwide field eyepiece (Say an Ethos or equivalent 100 deg AFOV).

The LGG 42 group appears to consist of 13 galaxies including NGC 750, NGC 751, NGC 761, NGC 777, NGC 783, NGC 785 and NGC 789 along with a few UGC galaxies. The group is also known as the NGC 777 group.

Visually both Night Sky Observer's Guide (NSOG) and Luginbuhl and Skiff (L&S) suggest that the 750/751 pair can be split with apertures in the 30-35cm range but obviously larger apertures will make this easier. Interestingly NGC 750/751 and 761 also feature in the Webb Deep-Sky Society Observer's Handbook (WSDSOH) Volume 4 where the suggestion is to see much detail you would need 16” (40cm) aperture although the 750/751 pair can be glimpsed as a single spot with a 8¾“ (21cm) telescope.

Owen Brazell - Galaxy Section Director