How are supermassive black holes formed?

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How are supermassive black holes formed?

Although supermassive black holes are called “black” holes, they are actually very bright.

In the middle of Delphinus and Pegasus, a “windmill” is slowly turning. For billions of years, the spiral arms of the UCG 11700 galaxy have been spinning peacefully, and have never collided with or merged with other galaxies, so they have not deformed like many galaxies. However, although this galaxy looks pleasing to the eye, there is a beast dormant in its center-there is a supermassive black hole, one of the most mysterious celestial bodies in the universe.

How are supermassive black holes formed?(1)

Virgo and other observatories The space-time ripples produced by black holes can be detected.

The mass of a general black hole is about four times that of the sun, but a supermassive black hole may reach millions, or even billions of times, that of the sun. Scientists believe that almost every large galaxy has a supermassive black hole in the center, but no one knows exactly how they formed.

In this regard, UCG 11700 may be able to help us.

“The ideal galaxy that I study is the most beautiful and perfect spiral galaxy you can think of,” said Becky Smallest, a junior assistant researcher at Oxford University who studies supermassive black holes. “These The most beautiful galaxies will help us solve the mystery of the growth of supermassive black holes.”

Black holes are so dense that even light cannot escape from them, so it is particularly difficult to study. But some new technologies can help us analyze the impact of supermassive black holes on the surrounding interstellar celestial bodies, and even the space-time ripples they produce, and provide us with new clues.

We have fully understood the formation and growth mechanism of traditional black holes. The star dies after running out of fuel, a supernova explodes, and then collapses inward. The density becomes so great that even light cannot escape. Almost a century has passed since Einstein predicted the existence of black holes in the general theory of relativity.

In popular culture, black holes are often described as extremely dark, extremely hungry images; they will move fast in the universe, swallow everything along the way, and continue to grow bigger and bigger. More and more greedy. Therefore, people will inevitably think that supermassive black holes are just the most thirsty and oldest kind of black holes. It’s that simple.

But in reality, black holes are actually a bit misleading. Their efficiency in absorbing the surrounding material is surprisingly low, even in the center of a densely packed galaxy. In fact, the collapsed star grows very slowly, and it is impossible to grow into a supermassive black hole by absorbing new matter.

“Assume that the first stars were formed about 200 million years after the Big Bang,” Smallster pointed out. “After they collapsed, it will take 13.5 billion years to grow to 50% of the mass of the sun. Million times. If you only rely on the quality of food, this period of growth is too short.”

How are supermassive black holes formed?(2)

Although the universe is expanding, galaxy collisions are actually quite common.

What’s even more puzzling is that supermassive black holes existed as early as the “infancy” of the universe. Distant quasars are one of the brightest celestial bodies in the night sky. They are actually dying galaxy nuclei lit by ancient supermassive black holes. These giant black holes have existed in the world at least when the universe was only 670 million years old. The oldest known galaxy also formed during this period.

Although the core of a black hole will always be a mystery to outside observers, the light of supermassive black holes is actually more dazzling than the stars of the entire galaxy combined, and they may even consume surrounding matter. “Hiccup”, releasing ultraviolet radiation.

The spherical boundary of a black hole is called the “event horizon”. Within this sphere, no light, energy, or matter can escape. Space and time are folded in layers, and common physical laws do not work here. But when the black hole rotates, it will also rotate the matter around the event horizon together, turning it into a rapidly rotating disk with extremely high temperature. The temperature of the accretion disk in a quasar can exceed 10 million degrees Celsius, emitting extremely bright electromagnetic radiation.

“Black holes are the most effective and efficient engine in the universe,” said Marta Volentelli, a black hole researcher at the Institute of Astrophysics in Paris. “They convert mass into energy with an efficiency of up to 40%. %. In contrast, the burning of fossil fuels and even the energy utilization of stars are only a fraction of black holes.”

Supermassive black holes attract scientists not only because of their super-high energy efficiency. Their formation and evolution are obviously closely related to the development process of galaxies, and even the history and structure of the entire universe. If the mystery of these giants of the universe can be solved, scientists’ understanding of the universe will also take a big step forward.

The release of energy is just one of the many ways that black holes “leak” their own secrets. When black holes merge or collide with other celestial bodies with lower density (such as neutron stars), they will form space-time ripples, which are what we call gravitational waves. Gravitational waves travel through the universe at the speed of light and were detected on Earth for the first time in 2015. Since then, LIGO in the United States and the Virgo Observatory in Italy have captured several time and space ripples created by such collisions.

How are supermassive black holes formed?(3)

The age of the universe It’s not big enough to make a black hole grow into a supermassive black hole just by absorbing the surrounding material.

However, although the detection instruments of these two observatories are several kilometers long, they can only detect gravitational waves produced by smaller black holes.

“In the merger event detected by LIGO, the mass of the black hole is at most 150 times that of the sun.” Nadine Neumeier, the leader of the Galaxy Nuclear Research Group at the Max Planck Institute of Astronomy, pointed out, “Currently , The so-called medium-mass black holes (that is, black holes with a mass of about 10,000 times the sun) are still lacking in relevant data. In fact, this type of black hole may become the seed that breeds supermassive black holes.”

Said that intermediate-mass black holes may be formed when giant gas clouds or stars collided with each other in the early universe. In the environment of frequent conflicts in the early universe, these medium-mass black holes may continue to collide with each other, and at the same time quickly absorb surrounding matter, thus rapidly growing to a super-mass scale.

However, this medium-mass black hole theory also has certain problems. The early universe was small and hot, and the gas cloud was always bathed in sufficient radiation. It stands to reason that the energy should be very sufficient, so that it would not collapse. And even in the dense early universe, the speed at which black holes can absorb matter is also limited by the laws of physics.

Warren Terry said that every current interpretation of supermassive black holes has “bottlenecks and defects”, so there is no final conclusion.

“These theories all mention what we call’dynamic processes’, that is, a black hole may be formed by multiple stars, not just one star, but these processes can only occur under limited conditions. ,” Warren Tly pointed out, “In addition, there is the theory of’primitive black holes’. It is believed that some black holes may have existed before stars appeared. However, this is still a completely unknown field, and there is no evidence of any observation results.” /p>

Warren Terry said that he appreciates the theory of dynamic processes, but he also admits that according to the theory, the mass of a black hole can hardly exceed 1,000 times that of the sun.

“Think about it, when the universe is only 1 billion years old, the mass of quasars is already 1 billion times that of the sun. Such a huge number is hard to reach.” Warrent Lee believes that the truth about the formation of supermassive black holes has yet to be revealed, “the deeper we dig down, the more problems we find. We must have missed something critical.”

Good The latest generation of observing instruments has begun to make up for the vacancy in this regard. Observatories such as Virgo and LIGO are providing us with more and more detailed “demographic information”, including the “body size”, age, and location of the black hole.

But in order to obtain data on supermassive black holes, researchers have to resort to larger detectors.

From 2030, NASA and ESA will jointly launch the Laser Interferometer Space Antenna Detector (LISA). The probe is composed of three satellites, distributed in a triangular shape, 2.5 million kilometers long on each side. The working principle of the satellite array is similar to that of LIGO and Virgo, but on a larger scale, it can detect gravitational waves emitted by supermassive black holes, which is beyond the reach of existing technologies.

There have been clues that the earth is being washed away by gravitational waves produced by supermassive black holes. In early 2021, astronomers announced that they had found some slight deviations in the radiation emitted by 45 pulsars. Although the results have yet to be confirmed, the researchers believe that these deviations may be caused by the “gravitational wave background” generated by the supermassive black hole.

But there are some more direct ways to help us see black holes. The Event Horizon Telescope recently took photos of black holes for the first time, taking off the mysterious cloak of such celestial bodies, and further revealing the nature of black holes and the influence of the gravitational and magnetism of black holes on the galaxy in which they are located. Astrophysicists can also track the trajectories of stars near the black hole in the core of the galaxy to infer relevant information about these giant celestial bodies.

Most of these observation activities are carried out by ground-based telescopes, and they all use the so-called “adaptive optics” technology. The observer measures the atmospheric distortion that affects the image quality by analyzing the bright stars (or artificial laser beams). The computer control signal then fine-tunes the shape of the telescope lens to correct these distortions. In this way, we can not only accurately observe the core of the galaxy billions of light years away, but also obtain a large amount of data related to supermassive black holes.

Neumayer was one of the first scientists to use adaptive optics to study the nucleus of galaxies. “It is hard to imagine that the resolution of observations from the Earth can exceed the Hubble Space Telescope.” Neumayer said, “I have been studying the mass of specific black holes and found that there is a close correlation between the mass of galaxies and the mass of black holes: The greater the mass of the galaxy, the greater the mass of the central supermassive black hole. These celestial bodies sometimes grow at the same pace.”

Although there is such a correlation, there is no clear evidence that large galaxies must Large black holes will be generated, and vice versa. There is indeed a connection between the two, but the nature of this connection is still a mystery.

There is an explanation that this may be related to galaxy collisions. There are approximately 2 trillion galaxies in the observable universe, most of which are accelerating away from each other, but galaxies may still collide, and the large black hole in the center will merge into a larger black hole. Some scientists believe that these terribly large supermassive black holes may be formed in this way.

When a relatively small black hole collides, it will release huge energy in just an instant, and everything in the space that is dazzling by the light will be eclipsed. And if two supermassive black holes collide, it is definitely one of the most catastrophic events in the universe.

However, although scientists suspect that supermassive black holes will also merge, due to a problem in the dynamics of black holes, the probability of this occurrence may be very low.

As the two black holes that are about to collide get closer and closer, they will rotate faster and faster around each other. But when the distance between large black holes reaches a parsec (3.26 light-years), their orbital speed will cancel out the gravitational force, causing them to approach each other at a greatly reduced speed. Judging from the current age of the universe, it is impossible to wait. The day when it really merged.

In spite of this, physicists still believe that this kind of merger will happen, but a new theory is needed to solve the so-called “last parsec problem.” Maybe there is some other force or energy that can draw two black holes orbiting each other together.

There are galaxies formed through mergers everywhere in the universe. Our Milky Way is one of them, indicating that this kind of event does happen. When galaxies collide, the stars, gas clouds, dark matter, and black holes in the two galaxies will interact, causing the original spiral structure to be destroyed. Even if two galaxies just pass by, it will affect the structure of both sides, so it is easy for us to discover.

But this also means that the supermassive black hole in the center of a galaxy like UCG 11700 cannot be explained by a galaxy collision, because the shape of the galaxy is so perfect that it has never been close to other galaxies. been touched.

“Only a very small number of galaxies have been’solitary’ since their birth, and have never touched other galaxies.” Becky Smotherst pointed out, “It is certain that the center of these galaxies The black hole of is never formed by merging with other black holes.”

This shows that they must have another way of formation.

Smotherst used the backward inference method to figure out how big these black holes needed to be at the beginning to grow to their current size. The model shows that black holes formed in the early stages of the universe with a mass between 1,000 and 100,000 times the sun have the greatest chance. This range coincides with Neumeier’s “Black Hole Seed Theory”. However, a black hole of this size is unlikely to be formed through star collapse.

There is also a theory that supermassive black holes may be directly formed by dark matter, and astrophysicists are exploring this possibility. Dark matter is a kind of theoretically existing particles. Although it can interact with gravity, it is completely invisible to light and electromagnetic waves, and our understanding of it is poor. If the two mysteries of black holes and dark matter are combined, it will only further increase the challenge.

“We don’t know a lot,” Smallest points out. “It’s too arrogant to say that black holes are all formed by supernova explosions. Maybe it’s true. Explain that we haven’t even thought of it. I look forward to the universe giving us a surprise one day, and it will be an important day for the entire scientific community.”

Next, more advanced observation instruments will appear soon. . NASA plans to launch the James Webb Space Telescope this fall. Its scale and detection capabilities have reached unprecedented levels, and it will surely become a valuable tool for studying supermassive black holes. Not only that, after the LISA antenna is launched into space, it will also provide scientists with a new way to observe supermassive black holes with gravitational waves.

Other scientists are beginning to draw detailed distribution maps containing information on the position, motion, shape, and size of millions of galaxies, which will also provide important assistance to observers and theorists.

“The pace of work in this field is really amazing,” said Smallest. “Our research on black holes has only been carried out for about 100 years, but compared with the 14 billion-year history of the universe, it is fundamentally Not enough to unravel all the mysteries. Every time a question is raised, at least five answers can be drawn.”

Newmayer also agrees with Smallest’s view that black holes may give us A huge surprise, far beyond everyone’s expectations.

“In the past century, we have made a series of technological developments that made these discoveries I want to learn about new things that I didn’t even think about. I think this is really great.” (Leaf)