Radio galaxy 0402+379 was formed as a result of the merger of a galaxy cluster. The two supermassive black holes at its center have been “dancing” around each other for several billion years. Scientists studied their motion and mass to understand what drives them to come together.

Almost every large galaxy has a supermassive black hole at its center. In 2004, scientists discovered two objects at the center of the radio galaxy B2 0402+379, 723 million light-years away from us (redshift 0.055). They immediately suggested that it was a double black hole. A few years later, the assumption was confirmed, astronomers even managed to detect the movement of two holes around each other. It was now possible to measure their masses.

Galaxies in which the radio emission is much stronger than other galaxies are called radio galaxies. Object 0402+379 is very bright indeed. It is a powerful elliptical galaxy surrounded by a bright X-ray halo. In terms of the number and total mass of stars, this single galaxy is comparable to an entire cluster.

Astronomers call such objects “fossil clusters” because they believe they are formed as a result of the merger of galaxies belonging to the same cluster. As for the supermassive black holes at the center, their merger, although theoretically predicted, is still unlikely and has never been encountered by astronomers during observations. The merger of stellar-mass black holes was first observed by scientists in 2015; This was the first gravitational wave signal ever detected in GW150914.

The supermassive black hole pair at the center of 0402+379 is the most compact of those directly measured. The objects are only 24 light years apart. As research has shown, they can withstand this distance for more than three billion years. To understand why, the authors of the new study investigated the dynamics of stars at the center of this galaxy. The results of their study were published in the journal Astrophysical Journal.

By comparing and analyzing data from the WIYN and Gemini observatories, scientists estimated the mass of a pair of supermassive black holes to be 28 billion solar masses. For comparison: The mass of the black hole at the center of the Milky Way is estimated to be 4.3 million solar masses. It turns out that the pair at the center of 0402 + 379 is 6.5 thousand times larger. This makes the pair the record holders in the modern universe, a universe about a billion light years away from us where the effects of cosmic evolution are minimal.

Despite such a large mass, this pair fits into the series of supermassive black holes at the centers of similar elliptical galaxies in all parameters, and their correlation is preserved, such as the correlation of the radius of the nucleus with the mass of the black hole. (or holes, as in this case).

Note that the supermassive black hole in the supergiant elliptical galaxy Holm 15A is theoretically even larger (about 170 billion solar masses, according to one paper), but its mass has not yet been measured. Therefore, for now, 0402+379 can be considered the record holder.

Why didn’t these holes meet? Apparently there is not enough mass, the “dance” of the pair “cleared” the central region of the galaxy from stars. Astronomers are lucky in this regard.

“Galaxies with less massive black hole pairs also appear to have enough stars and mass to force the pair together quickly. Here the pair is so massive that it takes a lot of stars and gas to do the job. But the pair still has enough stars and mass to force the pair together quickly.” “It cleared it of the relevant material, which allowed us to examine it.” explained Professor Roger Romani of Stanford University, one of the authors of the new paper.