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 arrived at the detectors of LIGO (the Laser Interferometer Gravitational Wave Observatory) and its Italian counterpart, Virgo. Scientists called the signal GW190412 and determined it emanated from a clash between two David and Goliath black holes, one three times as massive as the other. The signal marked the first detection of a merger between two black holes of very different sizes.
Now the new study, published today in the journal Physical examination letters, shows that this unbalanced merge can come from a very different process than how most merges, or binary, are supposed to form.
It is likely that the more massive of the two black holes was itself the product of an earlier merger between two parent black holes. The Goliath that emerged from this first collision may have ricocheted around a densely compacted “nuclear cluster” before merging with the second, smaller black hole, a raucous event that sent gravitational waves rippling through space. .
GW190412 may then be a second generation, or “hierarchical” merger, distinguishing itself from other first generation mergers that LIGO and Virgo have detected so far.
“This event is a strange event that the universe threw at us – it was something we didn’t see coming,” says study co-author Salvatore Vitale, assistant professor of physics at MIT and fellow of LIGO. “But nothing happens once in the universe. And something like that, although rare, we will see each other again and we can tell more about the universe. ”
Vitale’s co-authors are Davide Gerosa of the University of Birmingham and Emanuele Berti of Johns Hopkins University.
A struggle to explain
It is believed that black hole mergers form in two main ways. 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 ultimately share a common envelope, or gas disk. After a few billion years, black holes roll up and merge.
“You can think of it as a couple who’s been together all of their lives,” says Vitale. “This process is suspected to occur in the disk of galaxies like ours. ”
The other common path by which black hole mergers form is through dynamic interactions. Imagine, instead of a monogamous environment, a galactic rave, where thousands of black holes are crammed into 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 multiple times, before a pair of black holes eventually merge.
In the common envelope process and in the dynamic interaction scenario, the fusion black holes are expected to have roughly the same mass, unlike the unbalanced mass ratio of GW190412. They should also have relatively no spin, while GW190412 has a surprisingly high spin.
“Ultimately, these two scenarios, which people traditionally believe to be ideal nurseries for black hole binaries in the universe, struggle to explain the mass ratio and rotation 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 originate from a common envelope 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 the fusion of gas and working out how stars take shape and explode, then collapse into black holes that eventually merge. The second model simulates random and dynamic encounters in globular clusters – dense concentrations of stars around most galaxies.
The team ran the two simulations several times, adjusting the parameters and studying the properties of the black hole fusions that emerged. For mergers that formed through a common envelope process, a merge like GW190412 was very rare, only appearing after a few million events. Dynamic interactions were slightly more likely to produce such an event after a few thousand mergers.
However, GW190412 was detected by LIGO and Virgo after only 50 more detections, which suggests that it probably arose through another process.
“No matter what we do, we can’t easily produce this event in these more mainstream training channels,” says Vitale.
The hierarchical fusion process can better explain the unbalanced mass of 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 itself be more massive than either parent, and later would significantly outshine its first-generation partner, creating a high mass ratio during final melting.
A hierarchical process could also generate a merge with a high spin: the parent black holes, in their chaotic merger, would spin the resulting black hole, which would then cause that spin to be in its own ultimate collision.
“You do the math, and it turns out that the remaining black hole would have a turn that is very close to the total rotation of this fusion,” says Vitale.
If GW190412 was indeed formed by hierarchical fusion, 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 as a result of a previous collision, that collision likely produced a huge amount of energy that not only created a new black hole, but projected it onto a certain area. distance.
“If hit too hard, it would just leave the cluster and go into the empty interstellar medium, and couldn’t merge anymore,” says Vitale.
If the object were 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 it entered into. form. If GW190412 is indeed the product of a hierarchical merger, the team calculated that this would have happened in an environment with a escape speed greater than 150 kilometers per second. For perspective, the escape velocity of most globular clusters is about 50 kilometers per second.
This means that whatever environment GW190412 was born into had immense gravitational pull, and the team believe that such an environment could have been either the disk of gas around a supermassive black hole or a “cluster”. nuclear ”- an incredibly dense region of the universe, filled with tens of millions of stars.
“This merger must have come from an unusual place,” says Vitale. “As LIGO and Virgo continue to make new detections, we can use these findings to learn new things about the universe. “
Research where black hole binaries of very unequal mass originate
Davide Gerosa et al, Astrophysical implications of GW190412 as a remnant of an earlier black hole fusion, Physical examination letters (2020). DOI: 10.1103 / PhysRevLett.125.101103
Provided by the Massachusetts Institute of Technology
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