Black holes also come in pairs? The heaviest supermassive black hole pair ever discovered!

Using archival data from the Gemini North telescope, American astronomers have discovered the heaviest known supermassive black hole pair, which weighs 28 billion times the mass of the sun . This discovery helps scientists clarify a long-standing mystery: why supermassive black hole mergers are so difficult to occur in the universe. The relevant paper was published in the latest issue of The Astrophysical Journal.

01 The heaviest supermassive black hole pair

The research team analyzed a supermassive black hole pair located in the elliptical galaxy B2 0402+379. This is the only supermassive black hole pair that has been analyzed in sufficient detail, and the two black holes are only 24 light-years apart. The two black holes are so close that they may undergo a powerful merger. But further research shows that the pair of black holes have stayed at this distance for more than 3 billion years.

Roger Romani, a co-author of the paper and a professor of physics at Stanford University, said that to better understand the dynamics of this system and why it did not merge, they consulted archival data from the Gemini North Telescope to determine the speed of stars near the black hole, and from this they inferred that the total mass of the pair of black holes is 28 billion times that of the sun, making it the heaviest black hole pair measured to date.

The research team said that this measurement result 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 supermassive black hole pairs is the key to preventing them from merging.

Normally, galaxies with lighter black hole pairs seem to have enough stars and mass to allow the two to merge, such as the stellar mass black hole merger detected by scientists through gravitational waves in 2015. But because this pair of black holes is so heavy, it needs a lot of stars and gas to do the job. Given that there is not enough matter in the galaxy B2 0402+379 to do this, the merger of the black holes has stalled.

Romani noted that it remains to be determined whether the pair of black holes will eventually merge on a timescale of millions of years. If they do, the gravitational waves produced would be 100 million times stronger than a merger of stellar-mass black holes.

02 Merger of supermassive binary black holes

In fact, scientists have discovered more than once supermassive binary black hole systems that are about to merge, as well as some candidate bodies discovered through other means (mostly using the indirect effects caused by binary black holes). However, due to the uncertainty of observation, there is much controversy over whether these candidates are supermassive binary black holes. Moreover, these candidates are too far away from the earth, making it even more difficult to determine the distance between the binary black holes and the time of their merger. Scientists agree that the result of the merger of binary black holes is the birth of a supermassive black hole with a larger mass at the center of the galaxy, which is the main way for black holes to grow and form supermassive black holes.

Like the merger of stellar-mass binary black holes, the merger of supermassive binary black holes at the center of galaxies will also produce strong gravitational wave radiation. In 2016, the Laser Interferometer Gravitational-Wave Observatory in the United States directly detected the gravitational wave event GW150914 produced by the merger of stellar-mass binary black holes for the first time, opening a new era of gravitational wave astronomy.

The frequency of gravitational waves generated by the merger of supermassive binary black holes is not within the detection range of the ground-based gravitational wave detector Laser Interferometer Gravitational-Wave Observatory. For the merger of supermassive binary black holes with a mass of millions of solar masses, the frequency of gravitational waves emitted is in the millihertz band, which is the main detection target of future space low-frequency gravitational wave detectors, such as my country's "Tianqin" and "Taiji" and the European LISA project. The frequency of gravitational waves generated by the merger of binary black holes with a mass of hundreds of millions to billions of solar masses is at an extremely low frequency (nanohertz to microhertz), which can only be detected by pulsar timing arrays.

03 Space car accident scene

There are a lot of supermassive binary black holes in the universe, and many of the gravitational waves they generate are too weak to be distinguished. These signals are superimposed to form gravitational wave background noise. By detecting this gravitational wave background noise, scientists can estimate how many supermassive binary black holes are radiating gravitational waves to the Earth.

At present, astronomers have observed many "cosmic car crashes" of black hole mergers. This merger is the main way for galaxies to grow. Studies have shown that massive galaxies may be the result of multiple mergers of secondary galaxies. Mergers are also the reason for the formation of galaxy morphology. During the collision, the gas in the galaxy is compressed, causing a large number of stars to form rapidly, and starburst galaxies are thus formed.

At the same time, some galaxies with special shapes may also be the result of collisions. For example, the Cartwheel Galaxy may be the result of a small galaxy passing vertically through a large spiral galaxy.

Comprehensive sources: Xinhuanet, Science and Technology Daily, etc.