Morgan McFall-Johnsen
Thu, March 9, 2023
A potential supermassive black hole (bottom left arrow) leaves a trail of stars (middle arrow) as it's flung from a galaxy (top arrow).NASA/ESA/Pieter van Dokkum et al./Astrophysical Journal Letters 2023
A NASA Hubble image may show the first runaway supermassive black hole ever discovered.
A trail indicating an object traveling away from a galaxy hints that a black hole got kicked out.
A rogue black hole may have generated a shock wave that made a trail of new stars, visible in the image.
The Hubble Space Telescope is still making first-of-a-kind discoveries after more than three decades in space. Its latest? Observations of the first ever supermassive black hole gone rogue from its own galaxy.
That's what a team of astronomers is suggesting in a new study posted online. The study has been peer reviewed for publication in the Astrophysical Journal Letters, according to Pieter van Dokkum, an astrophysicist at Yale University who led the new study.
Even experts not involved in the study are excited for the team's results.
"The observations are all fitting together with this scenario," Manuela Campanelli, an astrophysicist at the Rochester Institute of Technology not involved in the study but who has simulated runaway black holes in her research, told Insider.
The first possible photo of a 'rogue' supermassive black hole
A trail of stars leaving a galaxy (the spot in the upper right of both images) and narrowing to a point on the lower left, indicating a runaway supermassive black hole.NASA/ESA/Pieter van Dokkum et al./Astrophysical Journal Letters 2023
What you're seeing above are two images of the same thing that tell the story of what happened.
Look at the zoomed-in shot on the right: The big spot in the upper right is a galaxy. Then follow the faint line trailing away from it, which ends in a point on the lower left. That's where scientists think the runaway black hole is hiding.
Black holes, by their very nature, are invisible. The reason astronomers are able to "see" any black hole is because it's surrounded by a swirling hot disk of gas, stars, and other cosmic stuff that is visible.
But the most fascinating part of these photos is the streak you see trailing behind the black hole. That's what caught researchers' eyes as they examined nearby stars.
They think that long tail coming out of the black hole is actually a trail of newborn stars, which formed after the black hole was hurled from its home galaxy, and ripped through space, generating a shockwave that caused clouds of intergalactic gas to collapse into stars.
"I thought that I'd actually made an error that there was this weird streak in the image," van Dokkum told Insider. "It didn't look like any astrophysical objects at first. And then it turned out that it was real. It was also in other datasets. And that's when I got excited."
Though black holes are notorious for devouring and destroying stars, this one seems to be creating them as well.
Further observations, probably with the James Webb Space Telescope, are necessary to confirm that the object in the picture truly is a runaway supermassive black hole.
Why a supermassive black hole would go rogue
Supermassive black holes are mind-bogglingly dense objects with the mass of billions of suns, and scientists think there's one at the center of every galaxy. Needless to say, kicking one out of its home would take a lot of force.
One such cataclysmic event that could possibly do the job is if two galaxies collide together, and their central black holes merge. A collision between black holes is one of the most violent, forceful events in the universe, and it could send a smaller remnant black hole careening into the void.
Astrophysicists have long theorized that black holes could "go rogue" or "run away," if other black holes pushed them out of their galaxies.
But nobody has ever confirmed a black hole wandering through intergalactic space, much less a supermassive black hole going rogue.
And while two galaxies colliding is the simplest explanation for a rogue black hole, that's not what seems to have happened here.
2 other black holes may have expelled this one in a rare, violent event
Van Dokkum thinks this black hole had an especially rare, dramatic, violent exit. Here's his theory: Two galaxies merged, and their supermassive black holes fell together, due to their sheer gravitational pull.
That happens all the time. Hubble has photographed plenty of merging galaxies, like the ones in the image below. The next step is what made this merger so weird.
NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and K. Noll (STScI)
The team thinks that a third galaxy arrived, with a third black hole, and its gravity caused a complex dance of the three black holes, which ended with ejecting one of them into the distance.
A merging pair of galaxies, captured by JWST.ESA/Webb, NASA & CSA, L. Armus, A. Evans
Ever since then, over a period of 39 million years, the runaway black hole has been screaming away from its home galaxy at a speed of about 1,600 kilometers (nearly 1,000 miles) per second, according to van Dokkum's team's calculations. For reference, at that speed it would take you 25 seconds to circle the entire Earth.
Basically, this supermassive black hole (if that's what it is) got third-wheeled and kicked out of its own home. Evidence for this third galaxy is yet to be confirmed, but the team is investigating a trail they see on the opposite side of the galaxy, where they think the other two black holes merged and then got kicked out by the recoil.
"The picture really tells the story," van Dokkum said.
That makes this event exceptionally rare, Campanelli said, because it involved three black holes instead of the conventional two that theorists typically pose in a scenario like this.
Follow the trail of newborn stars — if it's not just a jet
The other explanation for the mysterious trail in van Dokkum's Hubble photo is a fairly common one: jets of material that shoot out from the centers of galaxies with highly active black holes.
But van Dokkum and Campanelli both say that's unlikely, based on the shape of the trail in the new picture. Jets shooting from galactic centers fan out away from the galaxy, as the material shoots from a point and spreads out in the distance, like what's shown in the Hubble image below:
Spectacular jets powered by the gravitational energy of a supermassive black hole in the core of the galaxy Hercules A.NASA Goddard
Instead, the trail in van Dokkum's Hubble image fans out toward the galaxy. It seems to be a trail of new stars that formed as the traveling black hole generated shock waves in the intergalactic gas.
Campanelli added that the compact and irregular shape of the galaxy is "typical" of galaxies formed from mergers.
"If it turns out to be not real, I'll be surprised," van Dokkum said. "If it's not real, I think it is actually a combination of a few other gas clouds or something that seemed to line up in such a way that it looks like a streak."
Even though they're invisible, there's no reason to worry about rogue supermassive black holes sneaking up on us from other galaxies.
"We would have seen the effects of it if it was anywhere near us," van Dokkum said.
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