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There are many imaginary images of smaller black holes in the gas disk of supermassive black holes. Image source: J. Samsing / Niels Bohr Institute We have discovered supermassive black holes in the center of galaxies, double black holes orbiting and merging each other, and isolated black holes wandering in galaxies not long ago. This time, based on the gravitational wave event GW190521, astronomers speculate that there may also be a large number of smaller black holes on the gas disk around the supermassive black hole at the center of the galaxy.

Since September 14, 2015, there has been a fundamental change in the way humans look at the universe. Gravitational wave detectors led by LIGO and Virgo opened the era of multi-messengers in astronomy, through which humans can directly detect the craziest mergers between black holes and neutron stars, the densest celestial bodies in the universe.

The stronger observation ability not only answers many of our questions, but also brings more opportunities to science, the most obvious of which is to transform the "unknown unknown" into the "known unknown".

GW190521 is such a "known unknown". It was observed by LIGO and Virgo at 03:02:29 Universal time (UTC, approximately Greenwich mean time) on May 21, 2019, and scientists released the relevant observations on September 2, 2020. After analysis, the gravitational wave signal is generated by the merging of two black holes with 85 and 66 times solar mass respectively, resulting in a black hole with 142 times solar mass, of which 9 times solar mass is converted into gravitational wave energy.

The GW190521 waveform and time-frequency diagram observed by LIGO (the detector at Hanford on the left and the detector at Livingston in the middle) and Virgo (on the right). Photo: R. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration) the bizarre merger the merger was so strange that doubts about GW190521 even appeared in the Astrophysical Journal KuaiBao and Natural Astronomy.

First of all, the mass of these black holes is so strange. The black holes involved in the merger are too massive to be formed directly by stars. When stars below 130 times the mass of the sun die in the form of a supernova explosion, they can only form black holes of up to 50 times the mass of the sun. On the other hand, for stars with more than 130x solar mass, due to the high core temperature, high-energy gamma photons can even produce positive and negative electron pairs, directly reducing the thermal pressure of the core, leading to a violent birth pair unstable supernova (pair-instability supernova) explosion, the whole star will be blown up and nothing will be left. More massive stars can form black holes without experiencing supernova explosions, but such black holes are at least about 135 times the mass of the sun. Black holes between 50 and 135 times the mass of the sun are considered by scientists to be the product of the merger of other black holes. The black hole produced by this merger event is 142 times the mass of the sun, belonging to the medium-mass black hole that astronomers have been looking for. Astronomers have discovered many black holes, but they are all constant star black holes with several or ten times the mass of the sun, and supermassive black holes located at the center of the galaxy, often tens of millions or even tens of billions of times the mass of the sun, but very few medium-mass black holes with masses in between. Astronomers finally made up for this regret through GW190521.

The second is that the merger also produced a flash. Each time the gravitational wave signal is detected, the automated telescope will track and observe the corresponding direction under the guidance of the program. Generally speaking, black holes that can be swallowed by light do not glow during collisions. The scientists speculated that the flash was caused by the effect of the black hole merging on the surrounding dust.

The third strange point is that when black holes in GW190521 merge, their orbits are not exactly round. The analysis of gravitational waves shows that when the two black holes in GW190521 merge, their orbits are not standard asymptotic circular orbits, or the eccentricity (eccentricity) of the two orbits is not zero. "the basic properties of gravitational waves can not only shorten the distance between the two black holes to merge, but also make their orbits tend to be positive circles in the process." Explains Zoltan Hyman, a professor at Columbia University.

The first two anomalies can be explained, but the third anomaly is difficult to understand. it directly violates the model of the merger of two black holes-unless this merger is not a merger of two black holes at all, in addition to the two black holes involved in the merger. there are also "third-party" black holes affecting the "dance" of the merger of two black holes.

However, according to previous predictions, there should hardly be a "third party" when black holes merge in three-dimensional space. Almost all the gravitational wave events discovered by humans so far are in the following picture, and it is almost impossible to find the merger of three black holes in terms of probability. Scientists suspect that there is something wrong with the existing theory. It's like Xiaoming buys two different numbers of lottery tickets every day. Even if he wins once, he will feel lucky. But if one day he finds that his two numbers are the first prize and the second prize, he is more likely to inquire whether he is related to the boss of the lottery platform.

Blue indicates the black hole currently observed by gravitational waves, and the arrow indicates that two celestial bodies have been observed to merge and produce more massive objects. Photo: LIGO-Virgo / Aaron Geller / Northwestern University Black Hole Billiards Table and scientists have published their latest conjecture in the journal Nature to explain why we discovered the merger of the three black holes so early. They believe that the two merged black holes are actually located around a supermassive black hole, and there are many other small black holes around the supermassive black hole in addition to the gas disk produced by the accretion effect.

The situation described in this model is very similar to the galaxies full of black holes in the game "Stars". Source: Stellaris / paradox

By the way, the galaxies full of black holes in the game "Stars" are rich in dark matter. In some theories, black holes are indeed candidates for dark matter, but it is impossible to "collect" dark matter from black holes. Image source: Stellaris / paradox is like the solar system, with supermassive black holes in the middle and black holes taking the place of several planets. But these black holes do not have the exact repetition of the laws of most celestial bodies, their orbits are far from stable, around supermassive black holes, these "smaller" black holes are destined to be unstable.

"the latest research shows that gas disks play an important role in capturing smaller black holes, and over time, black holes get closer and closer. this means that not only some black holes will form black hole pairs, but also some black hole pairs may interact with a third black hole, which usually causes the three black holes to fly around in a chaotic dance." Zheng Yiming, an astrophysicist at Tohoku University in Japan, explained that he was involved in the study. So, this gas disk full of black holes is more like a billiard table, covered with "black hole billiards". Their trajectories are so chaotic and violent that they simply cannot be predicted, colliding and merging with each other from time to time, erupting huge gravitational wave energy.

Under this assumption, all weirdness becomes reasonable. It makes sense that the huge amount of energy released by black holes when they merge causes a flash of light in the gas disk. In this environment, black hole merging naturally occurs frequently, so it makes sense that the observed black hole mass is not within the range of stellar evolution, and it is entirely possible that they are formed by the merging of smaller black holes.

Moreover, from three-dimensional space compression to two-dimensional gas disk, the gap between black holes is much smaller, and the probability of black hole merging is greatly increased. "We have astonishingly found that the probability of eccentric merging of black holes has increased 100 times, and almost half of the merging of black holes in such a gas disk is eccentric." The author, Johan Samsing, an assistant professor at the Nils Bohr Institute at the University of Copenhagen, said: "this finding is very consistent with observations made in 2019, which now show that if it occurs on a flat gas disk around a supermassive black hole at the center of the galaxy, its otherwise amazing properties are no longer strange."

Chaotic stage astronomers also built a model of black hole merging in this case, and compared with the previous waltz in which black holes merge in an orderly way, the simulation of the "plot" of the merger of three black holes is still a bit bloodthirsty.

Schematic diagram of black hole merging in the gas disk of supermassive black holes. Source: Samsing, J. et al. Nature 603,237-240 (2022). Green, blue and purple represent the trajectory of a black hole, respectively. Green and blue represent a pair of black holes that revolve around each other, and in the absence of external interference, they slowly merge in a positive circle. But at this time, the black hole represented by purple suddenly burst in, and after a chaotic movement, whether out of the habit of "red and blue coming out of CP since ancient times" (forcibly seeing purple as magenta), or out of the spell of forgiveness color, the black hole represented by green was thrown aside. But its influence did not disappear completely. Under the influence of the black hole represented by green, the black hole represented by blue and purple also deviated from the positive circle during the merging, but merged with an elliptical orbit.

In addition to explaining the causes of strange gravitational wave signals, this model actually has unexpected uses for astronomers. According to the "unified model" proposed by astronomer Robert Antonucci (Robert Antonucci) in 1993, the supermassive black hole in the center of the galaxy with a gas disk is actually the active galactic nucleus (AGN). The active galaxy can be counted as the most energetic celestial body in the whole universe, and in some ways, it can even be brighter than the whole galaxy, which we call quasars. Astronomers have been very curious about the structure of its gas disk.

"for years, people have been trying to understand the structure of this gas disk, but the problem is difficult to solve. Our results are very sensitive to the flatness of the disk and the way the black hole moves in it. Time will tell us that once we observe more black hole merging events, including more unusual mergers like GW190521, we can improve our understanding of these disks. To that end, we must build on our existing achievements and see what new and exciting areas it will lead us to. " Professor Zoltan Hyman, co-author of the paper, concluded.

Reference link:

Https://www.eurekalert.org/news-releases/945806

Https://www.nature.com/articles/s41586-021-04333-1

Https://en.wikipedia.org/wiki/GW190521

Https://iopscience.iop.org/article/10.3847/1538-4357/ab584d

Https://www.nature.com/articles/s41550-021-01568-w

Https://iopscience.iop.org/article/10.3847/1538-4357/ab584d

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by Wang Yu, revised by Bai Defan

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