August 2016 - Galaxy of the Month

NGC 7727 in Aquarius

This interactive image of the NGC 7727 was provided by the Digitised Sky Survey using Aladin Sky Atlas. You can download a finder chart for this galaxy and another for the wider group.

NGC 7727, also known as Arp 222, is a face on spiral galaxy in Aquarius.

First discovered by William Herschel in 1785, the galaxy has numerous star streams and plumes associated with it which are probably the result of a merger with another spiral galaxy about 1 billion years ago. It may take another billion years for this merger to settle down. It was this odd shape that led to its inclusion in Arp’s catalogue of peculiar galaxies. The support for the merger scenario comes from the fact that there are two star like objects near the core of NGC 7727, one of which may be the core of the merging galaxy and the other the main core of NGC 7727.

NGC 7727 does not seem to have a large reservoir of hydrogen gas to form new stars so it is probably going to become an elliptical galaxy in the future. Images from GALEX, an ultraviolet satellite, show very little star formation going on at the current time in NGC 7727 compared to its neighbour NGC 7724.

Deep images such as those at Kent Biggs' website, the Sedona Stargazer Observatory gallery and captured by the Chilean Advanced Robotic Telescope 32 inch show the tails and plumes in the galaxy very well. The second link also shows an enhanced version of the core area which shows the dual cores as well.

NGC 7727’s classification of SAB(s)a pec also gives an idea of its jumbled state. The globular cluster system of NGC 7727 is also strange and it appears to contain 25 young globular star clusters which also support the merger hypothesis. Deep Hubble images show a string of dust clouds, perhaps remnants of spiral arms, in projection across the front of the galaxy. Hubble legacy images can be obtained from their website if you want to do your own processing.

NGC 7727 is also potentially part of a small group of galaxies listed as LGG 480, which also includes the nearby NGC galaxies 7723 and 7724, along with a couple of MCG galaxies, MCG-2-60-7 and MCG-2-60- 10. The reason I suggest that this group assignment may not be correct is that although NGC 7727 and 7723 are at approximately the same distance of 27 Mpc, NGC 7724 is listed at 37 Mpc in NED. The recession velocities however are fairly close.

The Night Sky Observer's Guide (NSOG) reports all the galaxies should be visible in a 20-22cm telescope but to see detail you may need 35cm telescope. NGC 7727 is also in the Herschel 400 list from the AL. A medium power field, say 200x centred on NGC 7727 should show NGC 7724 as well using a modern hyperwide eyepiece (100 AFOV). Modern visual observations of the galaxy can also be found on the Deep Sky Observer's Companion – the online database (DOCdb).

Owen Brazell - Galaxy Section Director