Arp-Madore 2339-661 was originally thought to be a pair of interacting galaxies, but is actually a trio consisting of NGC 7733, NGC 7734 and NGC 7733N. They are located in the constellation Tucana and are about to merge into a single entity. This image captured by the Hubble Space Telescope captures this pair of interacting galaxies, known as Arp-Madore 2339-661, so named because they belong to an unusual group of galaxies in the Arp-Madore cluster of galaxies.
This Hubble Space Telescope image of the galaxy Arp-Madore 2339-661 shows a pair of interacting galaxies in the swarm. However, upon closer inspection, we also discovered a third galaxy, making it three interacting galaxies. The main two galaxies are NGC 7733 and NGC 7734, while a third galaxy, NGC 7733N, looms in distinctive knot-like structures on NGC 7733's arms. Source: ESA/Hubble and NASA, J.Dalcanton, Dark Energy Survey/DOE/FNAL/NOIRLab/NSF/AURA, Credits: L.Shatz
This peculiar galaxy may be even stranger than first meets the eye, though, because there are actually three galaxies interacting here, not just two.
Two clearly visible galaxies are NGC 7733 (smaller, lower right) and NGC 7734 (larger, upper left). The third galaxy, currently known as NGC 7733N, can be spotted in this photo if you look closely at the upper arm of NGC 7733, where there is an obvious knot-like structure that is a different color than the upper arm and is obscured by dark dust.
The galaxy in the image could easily be considered part of NGC 7733, but analysis of the velocities involved in the galaxy (speed, while also taking into account direction) shows that this knot has a considerable extra redshift, meaning it is most likely its own entity and not part of NGC 7733. In fact, this is one of the many challenges that observational astronomers face: figuring out whether a celestial body is a celestial body from the perspective of the Earth, or is it an object lying in front of another celestial body?
These three galaxies are very close to each other, about 500 million light-years from Earth in the constellation Tucana, and, as this image shows, there are gravitational interactions between them. In fact, some scientific literature refers to them as "merging galaxy groups," meaning that they are moving toward eventually becoming a single entity.