NGC 672 in Triangulum
September 2024 - Galaxy of the Month
My challenge this month is the pair of galaxies in Triangulum, NGC 672 and IC 1727. NGC 672 was first discovered by William Herschel in 1786 and is an interesting galaxy which is part of an interacting pair with IC 1727. IC 1727 is much fainter than NGC 672 and was discovered photographically by Isaac Roberts in 1896. I am surprised that I have not covered them before.
The two galaxies are separated in space by perhaps only 88,000 light-years, which is about the diameter of one of the galaxies. The whole system is enveloped in a cloud of hydrogen gas and loose stars. There appears to be a tidal bridge between the two galaxies which shows up in radio observations made in the neutral hydrogen line (HI) at 21cm. The group lies less than 20 million light-years from the Milky Way.
Perhaps surprisingly given the obvious nature of the interaction is that NGC 672 did not make it into Halton Arp’s catalogue of peculiar galaxies, although it did make it into the Vorontsov-Velyaminov (VV) catalogue of interacting galaxies as number 338.
The colour of the galaxies is fairly blue due to all the star formation going on in them. Images from the UV satellite GALEX shows lots of evidence for new star formation. The total magnitude difference between the two galaxies is not that much, being 11.5 for NGC 672 and 12.1 for IC 1727, but the surface brightness is very different between the two which is why IC 1727 is so much more difficult to see. They are both barred spiral galaxies but IC 1727 is highly disturbed.
The pair appear to be part of a small group of galaxies which includes 4 dwarf galaxies as well. There is some suggestion that NGC 672 and its group of galaxies, along with the NGC 784 group, lie on a local dark matter filament which has been responsible for the growth of the galaxies.
Visually NGC 672 should be an easy target in a 20cm telescope but IC 1727 is likely to require a 30cm aperture or greater and a good night to find because of its low surface brightness. Using larger apertures on NGC 672 may show some of the large star clusters and other detail in its bar. As a bonus there is the nice edge on galaxy NGC 684 about half a degree to the NNE of NGC 672. This may be a challenge for larger telescopes.
NGC 672 makes it into the Astronomical Leagues Herschel II list as well as their Two in a View list. Along with the nearby NGC 684 it is also in Alvin Huey’s Galaxy Trios list.
Both of these galaxies appear in the Webb Society Deep-Sky Observer's Handbook (WSDSOH) Vol. 4 where it is suggested that they are both visible in a 25cm telescope, although IC 1727 was difficult. The The Night Sky Observer's Guide (NSOG) Volume 1 however suggests that a 30cm telescope is needed to see IC 1727 whilst NGC 672 can be seen with 15cm but the larger the telescope you use the more detail can be seen. Luginbuhl and Skiff (L&S) also suggest that IC 1727 can be seen in 25cm. You need to note however that all their observations were made at high altitude and will not reflect what can be seen with increased light pollution and crud with the extra depth of atmosphere over the UK.
Owen Brazell - Galaxy Section Director
