Astronomers have discovered the first solid evidence that merger events between black holes can deliver a “kick” powerful enough to spin a black hole out of its galaxy.
The team, which included Vijay Varma, a physicist at the Max Planck Institute for Gravitational Physics, Albert Einstein Institute, Germany, examined gravitational wave data from the merger event known as GW200129 collected by LIGO detectors and their European counterpart, Virgo. . Through that analysis, the scientists found that the black hole created in that collision and merger had been launched into space at 3 million mph (4.8 million km / h), a discovery described by one team member as “surprising and shocking.”
“When two black holes collide, they leave behind a more massive residual black hole. This process can impart a recoil ‘kick’ to the residual black hole,” Varma, lead author of a paper describing in detail the work of the team. .
Related: 8 ways we know black holes really exist
When black holes orbit each other, they emit gravitational waves – essentially gravitational radiation – which takes away energy and angular momentum as it ripples through the fabric of space. These emissions cause the orbit to shrink, leading to a collision and merger of black holes.
If black holes have unequal masses or spins, however, this leads to an asymmetry in the emission of gravitational waves, which are emitted mainly in one direction. Since the basic laws of physics require that momentum must be conserved, this asymmetry results in a large kick, which causes the residual black hole to back up in the opposite direction.
“Black hole mergers also emit gravitational radiation, similar to the astrophysical processes they emit electromagnetic radiation – light, “Varma continued.
These big kicks are expected when the orbital plane of the fusion precesses, or “wobbles”. Orbital precession is observable as a small change in amplitude in the gravitational wave signal. “This binary system of black holes is also the first sign to show strong signs of orbital precession, whereby the orbital plane wobbles,” coauthor Scott Field, a mathematician at the University of Massachusetts Dartmouth, told Space.com.
Varma added that by analyzing gravitational radiation, astronomers and astrophysicists can learn about black hole mergers. Furthermore, as black holes are influential in the evolution of galaxies, learning more about these processes could reveal how collections of stars such as the Milky Way develop.
This is the first time that astronomers have gathered strong evidence that such a merger can eject the resulting black hole from its galaxy.
(opens in a new tab)
“Unlike the previously observed black hole merger events, this is the first to provide strong evidence of the enormous recoil rate. Large enough, in fact, to allow the residual black hole to most likely escape from its host environment,” Field said. “Even though we knew that general relativity allowed such extreme possibilities in principle, we didn’t know if the universe would produce them. The velocity of the final black hole is large enough to most likely exceed the escape velocity of its host environment.”
Field added that this finding will also have important implications for binary black hole formation scenarios. This is because supermassive black holes, like Sagittarius A * (Sgr A *) in the heart of the Milky Way, are formed through a series of collisions that scientists call hierarchical mergers. Black holes driven out of a galaxy cannot participate in this process.
Mergers that give the boot to black holes
The discovery of fusions lopsided enough to give black holes a powerful kick is now possible thanks to technology that allows more precise detection of gravitational waves.
“Black hole mergers do not emit light, so gravitational waves are the only way to observe and learn about them. We would not know about this rogue black hole ejected without gravitational wave observers,” added Field.
Scientists are not exactly sure where the GW200129 gravitational wave event originated, so Field points out that the team cannot be completely sure that the black hole was ejected from its galaxy, but this is the likely result of his. movement at such extreme speeds, according to the researchers.
“If so, he now wanders the universe alone like a rogue black hole,” Varma said.
(opens in a new tab)
The merger that took place here could be a miniature version of an even more dramatic event, he noted. “A similar phenomenon occurs when supermassive black holes merge, which can happen after a galactic merger,” Varma said. “The last supermassive black hole can be displaced from, or even ejected from, the center of the merged galaxy, leaving behind a galaxy without a central black hole.”
Although existing gravitational wave detectors are not powerful enough to observe supermassive black hole mergers, the authors added that future space detectors such as the proposed Laser Interferometer Space Antenna (LISA) mission may be able to do so.
“Gravitational wave astronomy has produced many high-impact and truly extraordinary discoveries over the past five years or so,” Field said. “Before the first gravitational wave detection, the mantra of our field was that gravitational waves would open a new window to the universe. And this has proven true with each new LIGO observation run.”
The research is described in a paper (opens in a new tab) published May 12 in Physical Review Letters.
You can follow Rob Lea on Twitter at @ sciencef1rst. Follow us on Twitter @Spacedotcom (opens in a new tab) and go Facebook (opens in a new tab).