Hubble’s Galactic Reveal: A Dynamic Duo … or Trio?

This Hubble Space Telescope image of Arp-Madore 2339-661 reveals an interacting galaxy pair from the Arp-Madore catalog of peculiar galaxies. However, a closer look uncovers a third galaxy, making it a trio of interacting galaxies. The main two are NGC 7733 and NGC 7734, while the third, NGC 7733N, is subtly visible as a unique knot-like structure on NGC 7733’s arm.  Credit: ESA/Hubble & NASA, J. Dalcanton, Dark Energy Survey/DOE/FNAL/NOIRLab/NSF/AURA, Acknowledgment: L. Shatz

Arp-Madore 2339-661, initially thought to be a pair of interacting galaxies, is in fact a trio comprising NGC 7733, NGC 7734, and NGC 7733N. Located in the Tucana constellation, they are on course to merge into a single entity.

This striking image from the Hubble Space Telescope captures the interacting galaxy pair known as Arp-Madore 2339-661, so named because they belong to the Arp-Madore catalog of peculiar galaxies. However, this particular peculiarity might be even odder than first meets the eye, as there are in fact three galaxies interacting here, not just two. 

The two clearly defined galaxies are NGC 7733 (smaller, lower right) and NGC 7734 (larger, upper left). The third galaxy is currently referred to as NGC 7733N, and can actually be spotted in this picture if you look carefully at the upper arm of NGC 7733, where there is a visually notable knot-like structure, glowing with a different color to the arm and obscured by dark dust.

This could easily pass as part of NGC 7733, but analysis of the velocities (speed, but also considering direction) involved in the galaxy shows that this knot has a considerable additional redshift, meaning that it is very likely its own entity and not part of NGC 7733. This is actually one of the many challenges that observational astronomers face: working out whether an astronomical object really is just one, or one lying in front of another as seen from Earth’s perspective!

All three galaxies lie quite close to each other, roughly 500 million light-years from Earth in the constellation Tucana, and, as this image shows, they are interacting gravitationally with one another. In fact, some science literature refers to them as a ‘merging group’, meaning that they are on a course to ultimately become a single entity.