The center of most galaxies is surrounded by a giant supermassive black hole with masses ranging from millions or even billions of times to the mass of the sun. Current observations show that there is a supermassive black hole at the center of each galaxy, with no exception. They have different states – some are devouring matter, are active abnormally, some stop “eating” and sleep quietly.
However, a recent study found that there may be two supermassive black holes at the center of the galaxy.
Recently, an international team of researchers discovered periodic gamma-ray outbreaks in 11 active galaxies, two of which are known sources and nine are new sources. The researchers believe there may be a number of reasons for this phenomenon, one of which is the presence of two supermassive black holes at the center of these galaxies.
The findings have been published in the astrophysical journal.
Periodic change signals or important characteristics
It is well known that black holes have so much gravity that even light cannot escape their “magic claws”. How are supermassive black holes hidden in the core of galaxies found?
In fact, when a black hole is in an active “eating” state, the nearby matter rubs against each other during the process of being swallowed to release energy into a bright accretion disk, and some of the material may eject high-energy particles along the direction of the axis before being “eaten” to create a jet stream.
The scientists discovered these black holes hidden in the center of the galaxy by observing electromagnetic wave signals released by accretion disks or jets. “Typically, scientists can observe the black hole accretion disk through an optical band electromagnetic signal, and most of the energy from the jet stream is released in the form of gamma rays, X-rays, and radio radiation.” Lu Yijun, a researcher at the National Observatory of the Chinese Academy of Sciences, told Science daily.
In the study, the researchers used nine years of gamma-ray observations from NASA’s Fermi Space Telescope to systematically comb through the signals of active black holes at the centers of more than 2,000 galaxies, and eventually identified 11 gamma-ray sources with periodic patterns of change, with an average period of about 2 years.
“Determining the periodic patterns of gamma-ray signals is like finding tiny, regular ripples caused by a small boat passing by a stormy ocean.” The study may support the idea that there are two supermassive black holes at the center of the galaxy, said lead author Dr. Pernier of the University of Madrid in Spain.
Why does a binary black hole at the center of a galaxy cause periodic gamma-ray signals? Mr Lu said it could be understood in two ways.
On the one hand, the interference of the two black holes will cause periodic changes in the brightness of the jet stream. If there are two supermassive black holes at the center of the galaxy that orbit each other, the accretion disks around them will periodically “turn around” and the mass of matter that falls into the black hole will also have periodic changes that affect the brightness of the jet emitted by the black hole, and the telescope will receive periodic gamma-ray signals.
On the other hand, the rotation of the double black hole also causes periodic changes in the jet direction of the jet stream. Just as the Earth has rotations and rotations at the same time, in the dual black hole model, each black hole has orbital angular momentum that rotates around another black hole, and the rotation of its own rotation. In general, the orbital angular momentum and the rotational momentum are not the same, the two coupling, the rotational momentum will occur under the disturbance of the orbital angular momentum, the direction of the jet flow will change. Imagine that the direction of the jet flow of a single black hole is fixed, but due to the disturbance of another black hole, the originally fixed-direction jet stream swings back and forth around the center of the black hole in one cycle. Because telescopes on Earth are always in a particular direction of the target, the gamma-ray signals we receive are also strong and regular.
“In addition to gamma rays, there have been previous scholars who have used periodic light changes as a basis for observations of the existence of double black holes by combing the optical band signals emitted by the accretion disk.” Lu by jun said.
Of course, the periodic changes in the observed signals are not limited to double black holes, but also the intake of a single black hole accretion disk or jet stream, and quasi-periodic oscillations of the black hole’s accretion disk itself, which may also lead to similar periodic observations.
Early inferences but lack of observational “real hammers”
In fact, this is not the first time astronomers have mentioned the idea of a double black hole model, and scholars have speculated about the existence of supermassive double black holes at the center of galaxies decades ago, but have not found a “real hammer” in actual observations.
“The prevailing view in the academic world is that a collision or merger of two galaxies could lead to a supermassive double-black hole system at the center of the galaxy.” Lu said.
Astronomers have observed many previous collisions or mergers of galaxies, suggesting that this phenomenon is not uncommon, even more frequently in the early universe. For example, in a previous study published in the journal Nature, researchers found several examples of galaxy mergers at once, with black holes at the center of galaxies also close to each other. Although there is no real single-galactic binary black hole system, it will eventually form a double black hole after a long period of future evolution.
If there is a supermassive double black hole at the center of the galaxy, what effect will it have on the galaxy?
Lu told Science and Technology Daily that the double black holes circle each other at a higher angular speed, “kicking away” nearing the star, resulting in a lower density of the star at the center of the galaxy. In terms of observations, there will also be periodic changes in the relevant electromagnetic signals mentioned above, and the energy ratio and emission line profile changes in each band of the spectrum will also differ from that of a single black hole galaxy.
He further points out that the most “simple and crude” way to confirm the theory of a binary black hole at the center of a galaxy is to find, through astronomical observations, that there are two bright nuclei at the center of a single galaxy, two black holes, accompanied by typical black hole features such as accretion disks and jets.
On the other hand, the academic community speculates that supermassive double-black holes at the center of the galaxy radiate strong gravitational waves as they approach each other and merge. In 2016, astronomers detected for the first time gravitational waves from the merger of stellar-scale double-black holes. Of course, if it’s a supermassive black hole at the center of a galaxy, the gravitational wave energy released in the process of proximity and merging would be higher, but less frequent, than previously observed at the star-level mass of a binary black hole. One of the main goals of the space gravitational wave detection device, which is being planned in China, is to detect the integration of large-mass double black holes at the center of the galaxy.
“Unfortunately, so far there have been no cases of supermassive double black holes at the center of the galaxy that really convince everyone.” Lu said.
Also need to “shine eyes” patiently
In today’s rapid development of science and technology, why has not always found the existence of a double black hole at the center of the galaxy “real hammer”?
The first is because the telescope’s resolution is not high enough, Lu said. One possibility is that supermassive double black holes do exist at the centers of some galaxies we have observed in the adjacent universe, but limited by the resolution of telescopes, only one bright core has been presented, mistaken for a single black hole galaxy. So we also need a pair of sharper, brighter astronomical observation “eyes”.
Secondly, the time scale of human astronomical observation is far from sufficient. Since astronomer Galileo brought astronomy to the telescope era with a refractive telescope, modern astronomical observations have been in the hundreds of years at best. For humans, this may be a long time, but in the long river of the vast universe, more than a hundred years is just like a swing of the finger. “We may have missed a lot of ‘wonderful moments’ in the universe, so the evidence of a binary black hole at the center of a galaxy may require long waits and explorations in the future, such as detecting large-mass double-black holes and launching low-frequency gravitational waves.” Lu said.
However, even if we are very lucky, this speculation can be confirmed in the near future, this is not the end, but the beginning.
“Understanding the relationship between a binary black hole and its surroundings is critical to building a complete picture of the formation and evolution of galaxies.” Co-author Mark Aguero, an associate professor in the Department of Physics and Astronomy sciences at Clemson University in the United States, said.
Lu also pointed out that even if we clearly find evidence of a binary black hole at the center of a galaxy, we will continue to explore the mechanisms behind it about galaxy formation, evolution, and the characteristics of the double-black hole system, the merging, and so on.
“At that time, we can explore the nature of black holes in conjunction with the theory of electromagnetic waves and gravitational waves, so as to uncover more information about space-time and deepen our understanding of the nature of the universe.” Lu said.