It’s difficult, but try to imagine it. In the middle of the vast intergalactic black space, a huge supermassive black hole is fleeing alone at nearly a thousand kilometers per second. He is not at the center of a galaxy, but rather travels alone, wandering aimlessly and … ready to swallow anything that crosses its path. A huge shock arc precedes it, and a trail of star formation more than 200,000 light years long follows it like a tail, a bright scar that betrays its rapid escape. This is the first time that a similar phenomenon has been observed, and its discoverers, led by Yale University, have just described it in an article now available on the preprint server ‘arXiv’.
Until now, astronomers have been limited to observing “hypervelocity stars,” fast-moving suns violently kicked out of our Milky Way by powerful “gravitational kicks,” perhaps delivered by a collision with other stars or by the explosion of a nearby supernova. But what kind of “kick” does it take for an object like this to explode at 954 km/s? Because we’re talking about a “beast” with a mass equivalent to ten million suns, more than double that of Sagittarius A*, the black hole that reigns at the center of our own galaxy. And, at this speed, the object could travel the distance between Earth and the Moon in less than seven minutes.
As it escapes, the black hole compresses the gas and creates a “wake” of new stars 200,000 light years away.
The discovery, made thanks to the extraordinary infrared vision of the James Webb Space Telescope, confirms what was until now only a suspicion, first reported by the same researchers in 2023, when they first saw the object. Officially named RBH-1 (Runaway Black Hole 1), this enormous wandering black hole is located at a distance of 7.5 billion light years.
An unusual destination
Normally, supermassive black holes are associated with galaxies that grow and evolve around them dominated by their immense gravitational power. Therefore, the mere idea that one of these “black giants” could “pack its bags” and leave on its own flies in the face of both physics and our decades-long observational experience.
And yet, theory has been warning us for more than half a century that this could happen. “These results – write the researchers – confirm that the wake is caused by a supersonic supermassive black hole, a long-predicted consequence of the recoil of gravitational waves or the ejection of several bodies from galactic nuclei.” But it’s one thing to predict it on paper and another to see it with your own eyes.
Until now, astronomers had collected scattered clues. “Hyperspeed” stars ejected from the Milky Way had been detected, and even candidates for wandering black holes or galaxies that appeared to have a “hole” in their center, as if their “prime engine” had been ripped out. But what was missing was the definitive proof, the “body of the crime” that would demonstrate that something like this can happen in reality. And that’s exactly what RBH-1 represents.
Confirmation of result
To confirm that RBH-1 was what it appeared to be, astrophysicist Pieter van Dokkum and his team used James Webb’s NIRSpec instrument. Their goal was to analyze the speed of the gas around the object. And what they discovered was a “bow shock” structure at the front of the black hole. Something similar to water that collects and compresses into a wave in front of the bow of a ship traveling at full speed. Only, in space, the “water” is the circumgalactic medium, the tenuous gas and dust that exist in the apparent void between galaxies, and the “ship” is the black hole that rolls at high speed.
The object, named RBH-1, is moving so fast that it could cover the distance between Earth and the Moon in less than seven minutes.
Thanks to a happy coincidence, the entire structure is slightly tilted towards us, which allowed the researchers to measure the Doppler effect of the light emitted by the gas heated by the shock. Recall that, in the same way that an ambulance siren sounds higher as it approaches (due to the compression of sound waves) and lower as it moves away (due to its “stretching”), when a luminous object moves toward Earth, the light waves it emits compress and become bluer (blueshift) while as it moves away, they stretch and become redder (redshift).
So, by measuring this effect in front of and behind the crash arc, the team noticed an abrupt and significant difference in speed. In fact, the material located behind the shock front moves 600 kilometers per second faster than the material located in front, even if there is a minimum distance between them. Additionally, the gas at the outer edges moves away, showing a redshift. According to the study, this velocity signature is unequivocal: “RBH-1 – says the article – is an empirical validation of the 50-year-old prediction that supermassive black holes can escape from their host galaxies.”
Enormous strength
Which brings us directly to the big question: what kind of force is needed to move a mass of 10 million suns at 954 km/s?
In their 2023 paper, the researchers considered the possibility of a three-body interaction. That is, three black holes interact chaotically until one of them is kicked out, a space version of the famous game of chairs in which everyone except one manages to sit down when the music stops, but with cataclysmic consequences. However, with Webb’s new precision data in hand, the hypothesis evolved into an even more dramatic scenario, if possible: binary merger.
According to the new study, the most likely explanation is that two galaxies collided in the past, leading their respective central black holes to come very close to each other. So the two titans began to orbit each other, closer and closer until they merged into one. According to Einstein’s general relativity, when two black holes of unequal masses or different rotations merge, they emit gravitational waves (ripples in the structure of space-time).
But this emission is not symmetrical, that is to say it is not the same in all directions. It’s more like a cannon shot: if the energy is fired with more force in one direction, the object experiences a recoil, a “bounce” in the opposite direction. And in the case of RBH-1, this kick was so monstrous that it exceeded the escape velocity of the resulting galaxy itself. Thus emerges the newly formed black hole, condemned to wander forever in the intergalactic void.
star maker
It is paradoxical that, in its flight, during which the galaxy around it was probably destroyed, this black hole creates countless stars. And the immense pressure it exerts as it travels through the intergalactic medium causes, in its “tail”, authentic “vortexes” of gas, which collapse and ignite new suns. A brilliant “stele” 200,000 light years long transformed into a veritable nursery of stars, orphaned daughters of a father who will never stop meeting them.
This discovery not only solves a half-century-old mystery, but opens a new era in galactic archaeology.
Computer simulations suggest that the Universe could be filled with these invisible wanderers. For if, as we know, mergers between galaxies are, and have been, a very common phenomenon in the history of the Universe, then there should be a considerable number of “wandering supermassive black holes” lurking in the darkness, far from the heat of any galaxy, completely invisible to our telescopes until, like RBH-1, they interact with a sufficiently dense gas cloud.
This discovery therefore not only solves a half-century-old mystery, but also opens a new era in galactic archaeology. Now we know what to look for. The James Webb Telescope has taught us that the space between galaxies is not as empty as we thought, and that even monsters like RBH-1 can sometimes emerge from the darkness…
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