In what is being hailed as a "groundbreaking discovery", astronomers have for the first time observed two supermassive black holes orbiting around each other in a distant galaxy, according to new research.
In an article published in the Astrophysical Journal, researchers have detailed how they used radio telescopes to detect what appeared to be two black holes moving in relation to each other in radio galaxy 0402+379 .
"For a long time, we've been looking into space to try and find a pair of these supermassive black holes orbiting as a result of two galaxies merging," University of New Mexico's professor of physics and astronomy Greg Taylor said.
"Even though we've theorised that this should be happening, nobody had ever seen it until now."
The research team has been studying the two objects, which lie at the centre of the bulging galaxy, since 2003.
The galaxy itself was discovered in 1995 and is approximately 750 million light years away from Earth.
The lead author of the paper, Karishma Bansal, said the black holes are at a "separation of about seven parsecs," or 217 trillion kilometres.
"[This] is the closest together that two supermassive black holes have ever been seen before," she said.
The black holes are among the largest ever found, with a combined mass 15 billion times that of the sun, the study says.
If confirmed, it will be the smallest ever recorded movement of an object across the sky — at a rate of just over one micro-arc second per year, an angle about 1 billion times smaller than the smallest thing visible with the naked eye.
That means one black hole is believed to be orbiting around the other over a period of 30,000 years, the researchers said.
"If you imagine a snail on the recently discovered Earth-like planet orbiting Proxima Centauri — a bit over four light years away — moving at one centimetre a second, that's the angular motion we're resolving here," Stanford's professor of physics and co-author of the paper, Roger W Romani, said.
The researchers are hoping the finding will offer insight into "how black holes merge, how these mergers affect the evolution of the galaxies around them and ways to find other binary black-hole systems".
Large galaxies often have supermassive black holes at their centre and astronomers argue, if large galaxies combine, their black holes eventually follow suit.
As a result, the researchers have suggested that it is possible the apparent orbit of the black hole in 0402+379 is an "intermediary stage in this process".
But, given how slowly the pair is orbiting, the team thinks the black holes are too far apart to come together within the estimated remaining age of the universe, unless there is an added source of friction, they argue.