Using a worldwide network of telescopes, an international team of astronomers has discovered a pair of supermassive black holes less than 1 light-year apart.

The new discoveries, as reported in Nature Astronomy, are located in NGC 7674, a galaxy 400 million light-years from Earth. This is the second close pair of supermassive black holes to be recently discovered. They are more tightly bound than the previous pair, but also a lot lighter. The black holes in the first pair are 25 light-years apart and 15 billion times the mass of the Sun.

“The dual black hole we found has the smallest separation of any so far detected through direct imaging,” co-author Professor David Merritt, from Rochester Institute of Technology, said in a statement. “The combined mass of the two black holes is roughly 40 million times the mass of the Sun, and the orbital period of the binary is about 100,000 years.”

The observation is extremely exciting. The theory of black hole collisions is plagued by a big unknown – the so-called “final parsec problem”. Once two supermassive black holes are roughly 3 light-years apart (about a parsec) they don't have the means to lose orbital energy and collide with one another. We have indirect evidence that they do, but we don’t know how.

Follow-up observations will help us understand exactly how supermassive black holes merge. Although we have seen black hole collisions in gravitational wave detection from the Laser Interferometer Gravitational-Wave Observatory (LIGO), the release of gravitational energy is not enough to explain how these much bigger objects actually hit each other.

“A supermassive binary generates gravitational waves with much lower frequency than the characteristic frequency of stellar-mass binaries and its signal is undetectable by LIGO,” Merritt explained.

NGC 7674 was a selected target because it emits lots of radio waves, hinting that there's an active supermassive black hole at its center. No single telescope can resolve a potential pair of supermassive black holes, so researchers used different radio telescopes and connected them together. This specific technique, known as long baseline interferometry, created a “virtual” telescope that reached an angular resolution 10 million times better than the human eye.