NGC 3169 in Sextans

March 2024 - Galaxy of the Month

My choice this month is the galaxy trio in Sextans NGC 3169, NGC 3166 and NGC 3165. I can’t believe I have not covered this trio before so my apologies if I have.

NGC 3166 and NGC 3169 were discovered by William Herschel in December 1783 but it took until 1856 when Mitchell observing with the 72” at Birr found the much fainter NGC 3165. It appears that NGC 3166 and NGC 3169 are interacting strongly from the distortions seen in NGC 3169. The distance to all three galaxies is about 65 million light-years so it is likely that they form a physical group. It is probable that all three galaxies will eventually merge and for a single elliptical galaxy.

There is an excellent deep image of the group on Mark Hanson's website showing the tidal tails and interactions between NGC 3166 and NGC 3169. NGC 3165 is the bluer galaxy off to the right. There is also a fine image of the group taken with a 4m telescope at Kitt Peak. Images in the UV from GALEX show that NGC 3169 and NGC 3165 are showing signs of elevated star formation whilst NGC 3166 does not seem to show so much. Both NGC 3166 and NGC 3169 have active galactic nuclei of the LINER type. For a detailed view of NGC 3169 Hubble also shot the field.

The galaxies are also part of the galaxy group LGG 192 which contains 5 galaxies, including the three in this field and the nearby NGC 3156. They are also part of the larger Leo I group which is part of the Virgo cluster, see the Atlas of the Universe for more information on the Leo I group.

It is suggested that the activity in the central region of NGC 3169 is powered by a supermassive black hole. NGC 3166 and NGC 3169 are separated by only 160,000 light-years, so less distance than that separating us and the Magellanic clouds. NGC 3169 was also host to two supernovae in the last 25 years. Whilst NGC 3169 is classified as a spiral galaxy, albeit very distorted, NGC 3166 appears to be a lenticular with a smooth bar (SAB0(a)rs), although deep images do show some dust lanes. It also hosted a supernova in 2012. NGC 3165 is also classified as a spiral.

Both NGC 3166 and NGC 3169 appear in the Webb Society Deep-Sky Observer's Handbook (WSDSOH) Vol. 4 with observations with 16cm up to 40cm. The Night Sky Observer's Guide Vol. 2 suggests that 20-25cm will show NGC 3166 and NGC 3169 as elongated patches but not with much detail. They suggest that 40-45cm maybe needed to find NGC 3165. Luginbuhl and Skiff (L&S) suggest that NGC 3165 maybe glimpsed with averted vision with 25cm, however I suspect this was from a high altitude site. Hartung also briefly covers them suggesting that from his location they are right in 15cm (although this was rural Australia) and show in a nice star field.

For those that like collecting lists, NGC 3166 and NGC 3169 are in the Astronomical League's Herschel 400 list as well as Stephen O’Meara’s The Secret Deep book (numbers 40 and 41), along with many others.

Owen Brazell - Galaxy Section Director