A new study has uncovered an astonishing pair: two supermassive black holes, separated by a mere 100 parsecs (or 326 light years). In the grand scheme of space, this is quite close. The discovery was made in galaxy MCG-03-34-64, 800 million light years away. And the finding is surprising because of how close the two black holes are. Most dual black holes are either much farther apart or challenging to detect because they’re often obscured by dust and gas in late-stage mergers.

“We were not expecting to see something like this,” said Anna Trindade Falcão of the Center for Astrophysics, Harvard & Smithsonian in Cambridge, Massachusetts, and lead author of the paper published in The Astrophysical Journal.

This finding is among the closest dual active galactic nuclei (AGN) observed to date. Using an impressive array of tools, including the Hubble Space Telescope (HST), the Chandra X-ray Observatory, and the Very Large Array (VLA), researchers not only confirmed the presence of these two black holes but were also able to study their interactions and dynamics in unprecedented detail.

What’s going on in this galaxy?

MCG-03-34-64 is a gas-rich, infrared-luminous galaxy, sitting about 78 million light-years away, identified as one of the most intense X-ray sources in the local universe. This “local universe” is the group of galaxies in “our” corner of the universe, so to speak. This galaxy was already interesting for researchers due to its mysterious, heavily absorbed X-ray spectrum, indicating thick layers of material surrounding its central black hole. However, it was during routine observations with Hubble that the possibility of not just one, but two active nuclei came to light.

“This view is not a common occurrence in the nearby universe and told us there’s something else going on inside the galaxy,” said Falcão.

Most large galaxies host supermassive black holes at their centers, which are crucial to their evolution. These black holes, millions to billions of times more massive than the Sun, grow by drawing in gas and dust. So, their growth is tied to the galaxy’s development, influencing star formation. Meanwhile, they emit powerful energy as active galactic nuclei (AGN).

In the case of MCG-03-34-64, the discovery of two supermassive black holes suggests a galactic merger. Each galaxy likely brought its own black hole. As the galaxies merge, their black holes eventually combine, impacting the galaxy’s future. This dual system highlights how black hole mergers play a key role in the growth and evolution of galaxies.

Chasing Gravitational Waves

Understanding the dynamics of dual AGN systems is also key for gravitational wave astronomy. The eventual merger of these black holes will send ripples through spacetime, detectable by advanced instruments like pulsar timing arrays. These signals could unlock new insights into the physics of black holes and the fabric of the universe itself.

This type of binary AGN structure was much more common in the early universe when galaxy mergers were more frequent — or so we think. This discovery provides a unique close-up example to study. Previously, we identified dual AGNs with separations of a few kiloparsecs. But systems with sub-kiloparsec separations remained elusive. According to the study, the discovery of a candidate system with only 100 parsecs of separation is especially significant because it sets a new benchmark for such observations.

The two supermassive black holes are now no longer at the cores of their respective host galaxies. Their proximity draws them to one another, ultimately leading into a merger. Well, perhaps in around 100 million years. If this does happen, it will rattle the very fabric of space, sending forth gravitational waves.

The National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected gravitational waves from smaller black holes in the past. But the longer wavelengths resulting from a supermassive black hole merger are beyond LIGO’s capabilities.

Thankfully, researchers are already working on the next-generation gravitational wave detector, called the LISA (Laser Interferometer Space Antenna). The mission will consist of three detectors in space, separated by millions of miles, to capture these longer wavelength gravitational waves from deep space. ESA (European Space Agency) will be leading the mission, partnering with NASA and other participating institutions, with a planned launch in the mid-2030s.

Journal Reference: Anna Trindade Falcão et al, Resolving a Candidate Dual Active Galactic Nucleus with ∼100 pc Separation in MCG-03-34-64, The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad6b91