The many black hole mergers detected by gravitational-wave observatories have shown that black holes collide much more frequently than we thought. Now, new research suggests one way black holes can merge rapidly: they must be trapped in the accretion disk of a supermassive companion.
Astrophysically, getting two is relatively easy black holes close to each other. Either they are born as the offspring of one binary star system, or they meet by chance in the depths of interstellar space. And once in orbit, they can remain so and orbit each other endlessly.
Before that Detection of gravitational waves From black hole mergers, astronomers knew that black holes orbiting binary stars eventually met a fatal end. We know that large galaxies form from the collective merger of many smaller galaxies, and almost do every galaxy in the universe houses a giant black hole at its center. But usually everyone galaxy has only one giant black hole, which tells us that when galaxies merge, so must their black holes.
Related: Watch Two Monster Black Holes Merge into One in This Intricate NASA Simulation (Video)
But getting black holes to crash into each other is quite difficult. In order to bring two orbiting objects closer together, energy and angular momentum must be withdrawn from the system. Since space is pretty smooth, this is a tough task. Planetary systems do this all the time, but it usually involves the emission of radiation or the presence of gas. Black holes have no such option.
gravitational waves can pull energy and momentum from a binary system, but because gravitational waves are so weak, this process only works well when the two black holes are already very close together. This is known as the “last parsec problem,” where astronomers have recognized the difficulty of getting binary black holes close enough to let gravitational waves do the rest of the work to drive the merger.
Solution of the last parsec problem
Over the years, astronomers have devised a variety of ways to solve the last parsec problem. These mechanisms usually involved the presence of a third object far beyond the binary pair’s mutual orbit. When conditions are right, with just the right orientation and speed, the third companion can gently pull on the binary and expand its orbit. This stretching increases the ellipticity of the binary black hole’s orbit. As ellipticity increases, black holes begin to spend more and more time in close proximity. Once they reach a critical distance, gravitational waves close the gap and the black holes eventually merge.
However, this scenario requires precise configuration for the third companion, and that may not be enough. Based on all observations of gravitational waves, astronomers estimate that there are between 15 and 38 black hole mergers each year within each cubic gigaparsec volume in the universe (about 1/12000th of the total volume of the observable universe) and mechanisms that rely on a third companion, produce less than half that number.
In a new article published in the Preprint database arXiv (opens in new tab)researchers propose a new pathway for black hole mergers. It also uses a somewhat specific setup, but it’s much more general than depending on a distant third companion.
Construction begins near a supermassive black hole. This isn’t a crazy idea, since supermassive black holes sit at the centers of galaxies that are stuffed with Stars and with it many smaller black holes. As with most things near the center of the galaxy, we can likely find many binary black holes lazily orbiting the central supermassive hole.
The authors of the article discovered that if the binary is tilted with respect to its orbit around the supermassive black hole, there is a possibility of initiating a merger. First, the binary pair must precess, meaning that the axis of their orbital rotation wobbles over time. When the speed of this precession coincides with the orbital period around the central supermassive black hole, it is enormous heaviness will periodically tug on the binary, increasing its eccentricity and increasing the chances of an eventual merger.
But that kind of coincidence can only happen if the binary system’s orbit around the central supermassive black hole is just right, and that must be a very lucky coincidence. Fortunately, the authors found that nature can take care of that. Wherever the binary black holes are born, if they are in the supermassive black hole’s disk, they will slowly migrate inward.
Regardless of their initial orbit, they will eventually find a distance where their orbital period coincides with their precession period. If they stay in this orbit long enough, the central supermassive black hole’s gravity will have enough time to increase eccentricity.
The authors noted that this is a rather general scenario. By running many simulations of black hole properties and launch conditions, they discovered that binary black holes routinely merge.
It is not yet clear if this is the primary mechanism by which fusions occur or if it is just one of many ways. Regardless, the work shows just how complicated the lives of black holes can be and the many ways they can dance in the dark.
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