How and where in our universe do such black holes form and merge? Does it happen when nearby stars collapse and both turn into black holes, is it through close chance encounters in star clusters, or is it something else?
These are some of the key questions in the new era of Gravitational Wave Astrophysics,” says assistant professor Johan Samsing from the Niels Bohr Institute at the University of Copenhagen. Samsing and collaborators have now provided a new piece to the puzzle, which possibly solves the last part of a mystery that astrophysicists have struggled with for the past few years.
“For two black holes to merge within the age of the Universe, you need them to get really really close!” Samsing wrote to The Daily Galaxy in an email. “This can happen in many ways, e.g. in galactic nuclei where the density of stars can be million of times higher than our local solar neighborhood, or by the aid of a third object perturbing an already existing binary. But what about the black holes, how do they form? Maybe the heavier ones formed through collisions of many small black holes, or maybe they all formed at the big bang, so-called primordial black holes? We are at an extremely exciting moment in time, where we finally will start to find answers to these fundamental questions using data, namely gravitational wave data; Gravitational Wave Astrophysics is truly a field that will explode over the next years with the potential to deliver numerous breakthroughs in our understanding of our Universe.”
Why Do They Merge on a Non-Circular Orbit?
Researchers have provided the first plausible explanation to why one of the most massive black hole pairs observed to date by gravitational waves also seemed to merge on a non-circular orbit. Their suggested solution, now published in Nature, involves a chaotic triple dance inside a giant disk of gas around a supermassive black hole in another galaxy.
Black holes are one of the most fascinating objects in the universe, but our knowledge of them is still limited—especially because they do not emit any light. Up until a few years ago, light was our main source of knowledge about our universe and its black holes, until the Laser Interferometer Gravitational Wave Observatory (LIGO) in 2015 made its breakthrough observation of gravitational waves from the merger of two black holes.
Mystery Dates Back to 2019
The mystery dates back to 2019, when an unexpected discovery of gravitational waves was made by the LIGO and Virgo observatories. The event, named GW190521, is understood to be the merger of two black holes that were not only heavier than previously thought physically possible, but had also produced a flash of light.
A Third Astonishing Feature
Possible explanations have since been provided for these two characteristics, but the gravitational waves also revealed a third astonishing feature of this event—namely that the black holes did not orbit each other along a circle in the moments before merging.
“The gravitational wave event GW190521 is the most surprising discovery to date. The black holes’ masses and spins were already surprising, but even more surprising was that they appeared not to have a circular orbit leading up to the merger,” says co-author Imre Bartos, professor at the University of Florida.
“This is because of the fundamental nature of the gravitational waves emitted, which not only brings the pair of black holes closer for them to finally merge but also acts to circularize their orbit.” explains co-author Zoltan Haiman, a professor at Columbia University. Typically, gravitational waves first circularize the binary orbit well before the black holes merge. The discovery of a binary black hole that was still eccentric just a few orbits before merger was extremely surprising.
“It made me start thinking about how such non-circular (known as ‘eccentric’) mergers can happen with the surprisingly high probability as the observation suggests,” notes Johan Samsing.
Violent Centers of Galaxies Cancel Circular Binaries
A possible answer would be found in the harsh environment in the centers of galaxies harboring a giant black hole millions of times the mass of the sun and surrounded by a flat, rotating disk of gas.
“In these environments the typical velocity and density of black holes is so high that smaller black holes bounce around as in a giant game of billiards and wide circular binaries cannot exist,” points out co-author professor Bence Kocsis from the University of Oxford.
Chaotic Tango with Three Black Holes
“New studies show that the gas disk plays an important role in capturing smaller black holes, which over time move closer to the center and also closer to one other. This not only implies they meet and form pairs, but also that such a pair might interact with another, third, black hole, often leading to a chaotic tango with three black holes flying around, ” explains astrophysicist Hiromichi Tagawa from Tohoku University, co-author of the study.
Why Merge on a Weird Orbit?
However, all previous studies up to observation of GW190521 indicated that forming eccentric black hole mergers is relatively rare. This naturally brings up the question: Why did the already unusual gravitational wave source GW190521 also merge on an eccentric orbit?
Eureka Moment: The Flat Disk is Closer to a 2-D Environment
Everything that has been calculated so far was based on the notion that the black hole interactions are taking place in three dimensions, as expected in the majority of stellar systems considered so far.
“But then we started thinking about what would happen if the black hole interactions were instead to take place in a flat disk, which is closer to a two-dimensional environment. Surprisingly, we found in this limit that the probability of forming an eccentric merger increases by as much as a 100 times, which leads to about half of all black hole mergers in such disks possibly being eccentric,” says Johan Samsing and continues:
“And that discovery fits incredibly well with the observation in 2019, which all in all now points in the direction that the otherwise spectacular properties of this source are not so strange again, if it was created in a flat gas disk surrounding a supermassive black hole in a galactic nucleus.”
Solution Adds to a Century-old 3-Body Problem in Mechanics
“The interaction between three objects is one of the oldest problems in physics, which both Newton, myself, and others have intensely studied. That this now seems to play a crucial role in how black holes merge in some of the most extreme places of our universe is incredibly fascinating “, says co-author Nathan W. Leigh, professor at Universidad de Concepción, Chile.
Fits with Two Other Puzzling Properties
The theory of the gas disk also fits with other researchers’ explanations of the other two puzzling properties of GW190521. The large masses of the black hole have been reached by successive mergers inside the disk, while the emission of light could originate from the ambient gas.
“We have now shown that there can be a huge difference in the signals emitted from black holes that merge in flat, two-dimensional disks, versus those we often consider in three-dimensional stellar systems, which tells us that we now have an extra tool that we can use to learn about how black holes are created and merge in our universe,” says Samsing.
Baffling Structure of Gas Disks
“People have been working on understanding the structure of such gas disks for many years, but the problem is difficult. Our results are sensitive to how flat the disk is, and how the black holes move around in it. Time will tell whether we will learn more about these disks, once we have a larger population of black hole mergers, including more unusual cases similar to GW190521. To enable this, we must build on our now published discovery, and see where it leads us in this new and exciting field,” concludes co-author Haiman.
The Last Word – “Something Else?”
“Our understanding of the collisions of black holes underwent multiple transformations thanks to gravitational wave discoveries,” writes Batros in an email to The Daily Galaxy. “There was no observation of black hole collisions prior to 2015, making our knowledge of them very limited. The first discovery of a black hole collision by LIGO then came as a surprise, showing that such collisions are actually common in the Universe.”
“Then came the surprise of the discovery of GW190521 in 2019, which found a black hole more massive than what we thought possible. We thought black holes form when stars collapse under their own gravity, and this process is expected to put a limit of how big black hole can get, at around 50 times the mass of our Sun. One of the black holes in GW190521 was twice as massive as this limit.”
“Finally, we recently found that the orbit of the two black holes in GW190521 was likely eccentric, which was very surprising because gravitational wave emission makes orbits more circular. The heavy mass can be explained if the black holes are not from dying stars but themselves are the remnant of the collisions of smaller black holes.”
“Such consecutive collisions are actually uncommon in the disk-like gas surrounding supermassive black holes in galactic centers, called AGNs. AGNs act like black hole assembly lines, moving many black holes in a small volume (the inner part of the disk), where the chance of mergers becomes high.”
“As the new work shows, not only do AGNs produce heavier black holes, but they also make many of the collisions of these black holes eccentric. They seem to provide an explanation for all the peculiar properties of GW190521. So a uniquely interesting place in the universe where black holes can collide and develop unique properties is the dense disks of gas in the centers of galaxies around supermassive black holes.”
When asked how such very close binary black holes can form in an eccentric orbit, Zoltan Haiman, professor of astronomy at Columbia University, replied in an email to The Daily Galaxy: “The trick is for the two black holes to find themselves very close to one another. Gravitational waves are known to cause a pair of black holes to shrink their separation (“inspiral”) and eventually to collide and merge. However, for this process to work, the two black holes need to start from a distance that is already much smaller than the separation of the Sun and the Earth. This is unnatural and exceedingly rare.”
“An alternative possibility,” Haiman explains, “is for two stellar-mass black holes, initially separated far beyond this distance from each other, to find themselves embedded in the nearly flat gas disk swirling around the supermassive black hole in the nucleus of a distant galaxy. This gas disk is then able to exert a kind of a friction on the orbit of the two stellar-mass black holes, and cause their orbit to shrink — eventually gravitational waves can then take over and finish the job. This model serves as the starting point of the investigation in our newly published results — the additional key ingredients are that the pair of stellar-mass black holes, embedded in the gas disk, slowly “migrate” towards the supermassive black hole at the center of the galaxy. Because they are confined to the flat two-dimensional disk, they build up a very large density, making an encounter with a third black hole likely — this encounter can disturb the pair’s orbit and produce the apparent elliptical motion, seen in the LIGO data.”
Imre is the director of The Bartos Group at the University of Florida that focuses on multi-messenger astrophysics: the exploration of the Universe through combining information from a multitude of cosmic messengers, including gravitational waves, electromagnetic radiation, neutrinos, and atomic nuclei. Their research uses the LIGO gravitational-wave observatory, the future LISA gravitational-wave satellite, the IceCube neutrino observatory, the Karl G. Jansky Very Large Array radio observatory, the Fermi and Neil Gehrels Swift gamma-ray burst satellites, and others.
Source: Johan Samsing, AGN as potential factories for eccentric black hole mergers, Nature (2022). DOI: 10.1038/s41586-021-04333-1. www.nature.com/articles/s41586-021-04333-1