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An unbalanced merger of two black holes may have a strange origin story, according to a new study by researchers at MIT and elsewhere.
The fusion was first detected on April 12, 2019 as a gravitational wave that reached the detectors of LIGO (the Laser Interferometer Gravitational Wave Observatory) and its Italian counterpart Virgo. The scientists labeled the signal GW190412 and determined that it emanated from a collision between two David and Goliath black holes, one three times more massive than the other. The signal marked the first detection of a merger between two very different sized black holes.
Now the new study, published today in the journal Physical review letters, shows that this unbalanced merger may have originated through a very different process compared to how most mergers or binaries are believed to form.
It is likely that the more massive of the two black holes was itself the product of a previous merger between two parent black holes. The Goliath that emerged from that first collision may have bounced around a densely packed “nuclear cluster” before merging with the second, smaller black hole, a strident event that sent gravitational waves through space.
GW190412 can then be a second generation, or “hierarchical” merger, which is distinguished from other first generation mergers that LIGO and Virgo have detected so far.
“This event is a weirdo that the universe has thrown at us, it was something we didn’t see coming,” says study co-author Salvatore Vitale, an assistant professor of physics at MIT and a member of LIGO. “But nothing happens once in the universe. And something like this, although rare, we will see again and we can say more about the universe.”
Vitale’s co-authors are Davide Gerosa from the University of Birmingham and Emanuele Berti from Johns Hopkins University.
A struggle to explain
There are two main ways that black hole mergers are believed to form. The first is known as a common envelope process, where two neighboring stars, after billions of years, explode to form two neighboring black holes that eventually share a common envelope or disk of gas. After another few billion years, black holes spiral and merge.
“You can think of this as if a couple were together their whole lives,” says Vitale. “This process is suspected to occur in the disk of galaxies like ours.”
The other common path that black hole mergers form is through dynamic interactions. Imagine, instead of a monogamous environment, a galactic rave, where thousands of black holes huddle together in a small, dense region of the universe. When two black holes begin to associate, a third can separate the pair in a dynamic interaction that can be repeated many times, before a pair of black holes finally merge.
In both the common envelope process and the dynamic interaction scenario, the merged black holes should have roughly the same mass, as opposed to the unbalanced mass ratio of GW190412. They should also have a relatively zero effect, while GW190412 has a surprisingly high effect.
“The bottom line is that both scenarios, which people traditionally think are ideal nurseries for black hole binaries in the universe, struggle to explain the mass ratio and spin of this event,” says Vitale.
Black hole tracker
In their new paper, the researchers used two models to show that GW190412 is highly unlikely to come from a common enveloping process or dynamic interaction.
They first modeled the evolution of a typical galaxy using STAR TRACK, a simulation that tracks galaxies over billions of years, starting with gas melting and proceeding to the way stars take shape and explode, and then collapse into black holes. which eventually merge. The second model simulates random dynamic encounters in globular clusters, dense concentrations of stars around most galaxies.
The team ran both simulations several times, tuning the parameters and studying the properties of the black hole mergers that emerged. For those mergers that were formed through a common envelope process, a merger like GW190412 was very rare, emerging only after a few million events. Dynamic interactions were slightly more likely to produce such an event, after a few thousand merges.
However, LIGO and Virgo detected GW190412 after only 50 other detections, suggesting that it likely arose through some other process.
“No matter what we do, we cannot easily produce this event on these more common training channels,” says Vitale.
The hierarchical merge process can better explain the unbalanced mass of the GW190412 and its high spin. If a black hole were the product of a previous pairing of two parent black holes of similar mass, it would be more massive than either parent and then significantly outshine its first-generation partner, creating a high mass ratio in the final merger.
A hierarchical process could also generate a merger with a high spin: the main black holes, in their chaotic merger, would spin the resulting black hole, which would then lead this spin to its own final collision.
“You do the math, and it turns out that the remaining black hole would have a spin very close to the total spin of this merger,” explains Vitale.
There is no escape
If GW190412 was formed through hierarchical merging, Vitale says the event could also shed light on the environment in which it was formed. The team found that if the larger of the two black holes formed from a previous collision, that collision likely generated a large amount of energy that not only spun a new black hole, but kicked it some distance away.
“If you kicked it too hard, it would just leave the cluster and enter the empty interstellar medium, and it would not be able to merge again,” says Vitale.
If the object was able to merge again (in this case, to produce GW190412), it would mean that the kick it received was not enough to escape the star cluster in which it formed. If GW190412 is truly a product of hierarchical fusion, the team calculated that it would have occurred in an environment with an escape velocity greater than 150 kilometers per second. In perspective, the escape velocity of most globular clusters is about 50 kilometers per second.
This means that whatever environment GW190412 emerged from had immense gravitational pull, and the team believes that such environment could have been the disk of gas around a supermassive black hole or a “nuclear cluster,” an incredibly dense region of the universe, packed with of tens of millions of stars.
“This fusion must have come from an unusual place,” says Vitale. “As LIGO and Virgo continue to make new detections, we can use these discoveries to learn new things about the universe.”
