American astronomers using archival data from the Gemini North telescope have discovered the most massive pair of supermassive black holes known to date, with the two weighing in at 28 billion times the mass of the Sun. This discovery helps scientists clarify a long-standing mystery: why supermassive black hole mergers are difficult to occur in the universe. The related paper was published in a recent issue of The Astrophysical Journal.

The research team analyzed the supermassive black hole pair located in the elliptical galaxy B2 0402+379. This is the most thoroughly analyzed supermassive black hole pair to date, and the two black holes are only 24 light-years apart. The two black holes being so close suggests they may undergo a powerful merger. But further research showed that this pair of black holes has remained at this distance for more than 3 billion years.

Roger Romani, a professor of physics at Stanford University and co-author of the paper, said that to better understand the dynamics of this system and the reason it has not merged, they consulted archival data from the Gemini North telescope, determined the velocities of stars near the black holes, and thereby inferred that the total mass of this black hole pair is 28 billion times that of the Sun, making it the most massive black hole pair measured to date.

The research team said that this measurement not only provides valuable information for studying the formation of binary black hole systems and the history of their host galaxies, but also supports a long-standing theory that the mass of a supermassive black hole pair is the key factor preventing its merger.

Typically, galaxies hosting lighter black hole pairs appear to have enough stars and mass to allow the two to merge; for example, in 2015 scientists detected the merger of stellar-mass black holes via gravitational waves. But because this pair of black holes is very heavy, it requires a large amount of stars and gas to accomplish this. Given that the galaxy B2 0402+379 lacks sufficient material to do so, the merger of this black hole pair has stalled.

Romani noted that whether this pair of black holes can ultimately merge on a timescale of millions of years remains to be determined. If they merge, the intensity of the gravitational waves produced would be 100 million times that of the merger of stellar-mass black holes.