There are many images of smaller black holes in the gas disk of a supermassive black hole. Image credit: J. Samsing/Niels Bohr Institute
We’ve found supermassive black holes at the center of galaxies, we’ve found pairs of black holes orbiting and merging with each other, and just a while ago, we found isolated black holes wandering through galaxies. And this time, astronomers have guessed from the gravitational wave event GW190521 that there may also be lots of smaller black holes in the gas disk around the supermassive black hole at the galaxy’s center.
Since September 14, 2015, the way we observe the universe has fundamentally changed. The gravitational wave detectors, led by LIGO and Virgo, ushered in the era of multi-messenger astronomy, allowing us to directly detect the craziest mergers between the universe’s densest objects: black holes and neutron stars.
While greater observational capacity answers many of our questions, it also opens up more opportunities for science, most notably the transformation of “unknown unknowns” into “known unknowns.”
GW190521 is such a “known unknown”. It was observed by LIGO and Virgo at 03:02:29 Coordinated Universal Time (UTC) on May 21, 2019, and released on September 2, 2020. After analysis, the gravitational wave signal was generated by the merger of two black holes, each with a mass of 85 and 66 solar masses, respectively. The merger created a black hole with a mass of 142 solar masses, of which 9 solar masses were converted into gravitational wave energy.
Waveforms and time-frequency plots of GW190521 observed by LIGO (Hanford detector, left, Livingston detector, center) and Virgo (right). Image credit: R. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)
The merger was so bizarre that doubts about GW190521 were even published in the astrophysical Journal Letters and Nature Astronomy.
The first is that the masses of these black holes are so weird. The black holes involved in the merger are too massive to form directly from a star. When a star less than 130 times the mass of the Sun dies in a supernova explosion, a black hole of 50 times the mass of the sun can only form. For stars above 130 times the mass of the Sun, the core is so hot that high-energy gamma photons can produce pairs-plus and minus electrons, directly reducing the thermal pressure in the core and causing a pair-instability supernova that can blow the entire star to dust. There will be nothing left. More massive stars can form black holes without going through supernova explosions, but the smallest such black holes can be about 135 solar masses. Black holes between 50 and 135 solar masses appear to be the result of mergers between other black holes. At 142 solar masses, the black hole created by this event is one of the intermediate-mass black holes astronomers have been looking for. Astronomers have found a lot of black holes, but they are about a few, ten times the mass of the sun’s stellar black hole, and in the center of the galaxy, often millions to tens of billions of times the mass of the sun’s supermassive black hole, but rarely found in the middle of the mass of the black hole. Astronomers have finally filled in the blanks with GW190521.
Second, the merger produced a flash of light. Each time a gravitational wave signal is detected, an automated telescope is programmed to track it in the right direction. Normally, black holes that can swallow even light don’t glow when they collide. Scientists speculate that the flash was caused by the impact of the black hole merger on the surrounding dust.
The third oddity is that the black holes in GW190521 had a perfectly circular orbit when they merged. Analysis of gravitational wave forms has shown that the two black holes causing GW190521 did not have a standard asymptotic circular orbit, or zero eccentricity, when they merged. “The fundamental properties of gravitational waves not only bring the two black holes closer together to merge, but also make their orbits tend to be perfectly circular in the process.” Explains Zoltan Haiman, a professor at Columbia University.
The first two anomalies can be explained, but the third is hard to understand and directly contradicts the black-hole merger model — unless, of course, the merger wasn’t a black-hole merger at all, and there were other black holes affecting the merger dance.
However, according to previous predictions, there should be almost no third party when black holes merge in a three-dimensional universe. Almost all of the gravitational wave events detected so far are shown in the chart below, making it almost impossible to find a triple black hole merger. Scientists suspect something is wrong with the existing theory. It is like xiao Ming would buy two different lottery tickets every day, even if he won once, he would feel lucky. But if one day he finds himself writing two numbers one is the first prize, one is the second prize, then he is more likely to inquire whether he has any kinship with the boss of the lottery platform.
The blue shows black holes currently observed with gravitational waves, and the arrows show two objects merging to create a more massive object. Image credit: LIGO-Virgo/Aaron Geller/Northwestern University
Black hole pool table
And scientists have just published their latest theory in the journal Nature, which could explain why we’ve seen these three black hole mergers so early. They suggest that the two merging black holes are actually located around a supermassive black hole, and that there are many other smaller black holes around the supermassive black hole besides the gas disk produced by accretion.
This model describes a situation very similar to the black hole filled galaxy in the game Stars. Image credit: Stellaris/ Paradox
Incidentally, the black-hole galaxy in the game Stars teems with dark matter. Black holes are indeed candidates for dark matter in some theories, but it is impossible to harvest dark matter from black holes. Image credit: Stellaris/ Paradox
Just like the solar system, the supermassive black hole is in the middle, and the black hole takes the place of several planets. But these black holes don’t have the precise repetition that most celestial bodies do, and their orbits are far from stable. Around supermassive black holes, these “smaller” black holes are destined for unstable orbits.
“The latest research suggests that gas disks play an important role in trapping smaller black holes, bringing them closer together over time. This means that not only do some black holes form black hole pairs, but some may interact with a third black hole, which usually causes all three to fly around in a chaotic dance.” Astrophysicist Shengming Zheng of Tohoku University in Japan, who was involved in the study. So the black-hole-filled disk of gas is more like a pool table, filled with “black hole balls” that move in chaotic, violent, unpredictable paths, occasionally colliding with each other and erupting in huge bursts of gravitational wave energy.
Under this assumption, all the weirdness is justified. It makes sense that the energy unleashed by the black hole merger would cause a flash of gas in the disk. Black hole mergers happen frequently in such environments, too, so it makes sense that the observed black holes’ masses are outside the bounds of stellar evolution, rather than being formed by successive mergers of smaller black holes.
And, as the space compresses from three dimensions to two dimensions, the space between the black holes becomes much smaller and the probability of black holes merging is greatly increased.” “What we found was a remarkable 100-fold increase in the probability of black hole mergers being eccentric. Almost half of these black hole mergers in such gas disks are eccentric.” Author Johan Samsing, assistant professor at the Niels Bohr Institute at the University of Copenhagen. This finding fits in nicely with 2019 observations, which now suggest that if it happens in a flat disk of gas around a supermassive black hole at the center of a galaxy, its otherwise surprising properties don’t seem so strange anymore.”
Astronomers have also modeled black hole mergers in this scenario, and compared with the orderly spiral waltzes of previous black hole mergers, the scenario looks a bit silly.
A schematic of black hole mergers in a supermassive black hole gas disk. Source: Samsing, J. et al. Nature 603, 237 — 240 (2022).
Green, blue and purple represent the trajectory of a black hole. Green and blue represent a pair of black holes that were originally orbiting each other, slowly merging in a perfectly circular orbit like this without interference. But then the black hole of purple burst in, and after a flurry of movement, the black hole of green was tossed aside, either by the habit of forcing purple to be magenta, or by the curse of forgiving color. But its influence has not completely disappeared. Under the influence of the black holes represented by green, the orbits of the black holes represented by blue and purple also deviated from the perfect circle, but merged in an elliptical orbit.
In addition to explaining strange gravitational wave signals, the model actually has unexpected uses for astronomers. Supermassive black holes with gas disks at the center of galaxies are known as active galactic nuclei (AGNs), according to the 1993 “Unified model” proposed by astronomer Robert Antonucci. Active galaxies account for the highest energy objects in the entire universe. It can even outshine an entire galaxy and is called a quasar. Astronomers have long been curious about its gas disk structure.
“People have been trying to understand the structure of this gas disk for years, but the problem has been difficult to solve. Our results are sensitive to the flatness of the disk and the way the black hole moves in it. Time will tell if we can improve our understanding of these disks once we observe more black hole mergers, including more infrequent ones like GW190521. To do so, we must build on what we have already achieved and see what new and exciting areas it leads us into.” Concludes co-author Professor Zoltan Hyman.