A rare triple active galactic nucleus

Image: ESA/Hubble and NASA/J. Dalcanton/Dark Energy Survey/DOE/FNAL/NOIRLab/NSF/AURA.

This cosmic collision is a coming together of two large spiral galaxies, but if you look carefully you might see two smaller galaxies that are also involved. The large galaxy in the top half of the picture is NGC 7734, while the other, distorted, galaxy is NGC 7733. Both have had their spiral arms unwrapped, with NGC 7733 in particular forming a warped ring galaxy with a lens or bar in the middle. On NGC 7733’s blue ring of stars is a noticeable bright knot. This is a third galaxy, NGC 7733N, which lies between us and NGC 7733. It’s interacting with NGC 7733, and may be partially to blame for the ring structure (this normally forms when a smaller galaxy passes through a larger galaxy). As for the fourth galaxy, it is the sliver of light to the lower-left.

Ultraviolet observations suggest that it is connected to NGC 7733 by a river of hot gas, although this has yet to be confirmed via redshift measurements. All four galaxies, which are 500 million light years away, are interacting and will eventually merge to form one giant elliptical galaxy. In 2021 a study published in the journal Astronomy and Astrophysics, using data from the Very Large Telescope’s MUSE (Multi Unit Spectroscopic Explorer) instrument and near-infrared observations from the South African Astronomical Observatory, indicated that NGC 7733b contains an active galactic nucleus (AGN) called a Low Ionisation Nuclear Emission-line Region (LINER), while both NGC 7734 and NGC 7733N contain Seyfert AGN. That makes this system, collectively known as Arp–Madore 2339-661, a rare triple AGN. When the galaxies all merge, the supermassive black holes at the heart of these AGN will also merge.

This image of the galaxies is a composite taken by the Hubble Space Telescope’s Advanced Camera for Surveys and the Dark Energy Camera on the Victor M. Blanco four-metre telescope in