A supermassive black hole, booted out of its galaxy by powerful gravitational forces, is leaving a trail of ionized gas and newborn stars in its wake.

Galaxies colliding and merging is a pretty common event in the universe. Most of the larger galaxies we see today formed in a series of galactic mergers, and our own spirally home will eventually smash into the Andromeda Galaxy. When galaxies merge, their supermassive black holes usually merge, too; drawn by each other’s gravity, the two supermassive black holes end up orbiting each other in a death spiral until they finally collide and become one, sending powerful gravitational waves out into the universe in the process.

But a tiny fraction of the time, computer simulations suggest, things get weird, and one supermassive black hole gets booted out of the galaxy. Yale University astronomer Pieter van Dokkum and his colleagues may have just found evidence of that happening in an oddly-shaped little galaxy about 10 billion light years away.

They describe their findings in a recent pre-print paper.

Finding the invisible

In a recent batch of images from the Hubble Space Telescope, van Dokkum and his colleagues noticed a narrow streak on one side of a small galaxy. The galaxy doesn’t even have a catalog number yet, and it’s relatively small, but it’s densely packed with gas clouds and teeming with new star formation. From its center, a nearly straight streak — visible because it’s much dimmer than the rest of the galaxy — extends out more than 200 light years, where it ends in a bright blaze of ultraviolet light.

“Not having encountered something quite like this before in our own images or in the literature,” they write in a recent paper, van Dokkum and his colleagues booked some time with the Keck Observatory to take another look.

Along the 200 light year-long streak, clumps of newborn stars blaze with ionizing radiation. By measuring the wavelengths of light from the stars, van Dokkum and his colleagues could estimate their age; the youngest stars are the ones farthest from the center of the galaxy. And van Dokkum and his colleagues say it looks as if a powerful shock wave barreled outward from the galaxy’s core, carving a path for itself and leaving a trail of hot, compressed gas that triggered bursts of star formation in its wake.

The explanation that best matches the data from Hubble and Keck is something truly wild: a runaway supermassive black hole. As van Dokkum and his colleagues suggest, about 39 million years ago, a galactic merger kicked a supermassive black hole out of the galaxy’s core and flung it out into space at about 360,000 miles per hour.

How the engines of a black hole turn

Typically, when two supermassive black holes merge, they form an even bigger black hole — let’s call it a superdupermassive black hole — which sits at the heart of the newly-formed galaxy. But once in a while, the gravitational waves from the merger create a recoil, which gives the new superdupermassive black hole a powerful kick that sends it careening outward from the galaxy’s core.

When that happens, you end up with an incredibly massive, incredibly dense object moving at terrifying speeds through clouds of interstellar gas, pushing a bow wave ahead of it and trailing a long wake of ionized hydrogen, ripples of compressed gas, and bursts of newborn stars behind it.

And van Dokkum and his colleagues think that may be what’s happening in their unnamed dwarf galaxy.

Digging into the details

The idea gets some support from the surprising lack of activity in the little galaxy’s center. There’s no actively feeding supermassive black hole there, which you’d expect to find if two supermassive black holes had just merged. And that, according to van Dokkum and his colleagues, “reflects the departure of all supermassive black holes from the nucleus.”

So what happens to a galaxy with no supermassive black hole at its center? Not much, according to Georgia State University astronomer Misty Bentz, who was not involved in the study.

“A galaxy without a black hole at its center would look the same, in most respects, as a galaxy with a black hole,” she tells Inverse. “The stellar mass of a galaxy is, on average, 1000 times larger than the mass of the black hole at the center, and there is an even more massive dark matter halo within which a galaxy is embedded. So while a typical galaxy appears to be orbiting around the supermassive black hole at its center, it is really orbiting around its center of mass, and the black hole just happens to be sitting at the gravitationally stable point that is the center of mass.”

However, the stars orbiting closest to the supermassive black hole — the bright denizens of the galactic nucleus — would definitely feel some effects from the cataclysmic merger and sudden ejection of their supermassive neighbor. In fact, in some simulations, a few of those stars are so tightly bound to the supermassive black hole by gravity that when it leaves, they leave, desperately holding on for the ride.

And van Dokkum and his colleagues say they’ve spotted two objects that might be clusters of those very attached stars — one near the bright knot of light where the wake ends, and another on the far side of the galaxy.

Plot twist

Why the far side of the galaxy? Because this nameless little galaxy may have kicked out not just one, but two supermassive black holes. In their data, van Dokkum and his colleagues saw “a fainter and shorter feature” visible in the shortest ultraviolet wavelengths, called UVC.

“The feature may be shocked gas behind a binary supermassive black hole that was ejected at the same time as the supermassive black hole that produced the primary wake,” they write. That scenario involves a three-way galactic merger, like the one happening right now in Stephan’s Quintet.

“If a third supermassive black hole reaches the center of the galaxy before the binary merges, a three-body interaction can impart a large velocity to one of the supermassive black holes, leading to its escape from the nucleus,” write van Dokkum and his colleagues. And in that case, the supermassive black hole may actually be boosted to escape velocity, sending it flying clear out of the galaxy.

Where does it go from here?

“It’s an intriguing source, and the wake of a runaway black hole is a potential (and exciting!) interpretation,” says Bentz. “However, there are other possible interpretations. [van Dokkum and his colleagues] are pretty careful to say that this *may* be an example of a runaway black hole. They also clearly describe how to improve on the observations that were presented in this paper. So I think we will have to wait for further data in order to figure out what is happening here.”

The most important step will probably be to see what the galaxy looks like in X-ray radiation, which will likely be a job for NASA’s Chandra X-ray Observatory. If astronomers get extremely lucky, Chandra could even catch a glimpse of the accretion disk around the runaway supermassive black hole — or even both of them, if there actually are two. That would be “the ‘smoking gun’ evidence for this scenario,” as van Dokkum and his colleagues put it.

And more images in the shortest ultraviolet wavelengths, called UVC or deep ultraviolet, could show astronomers whether that second, much fainter line is a second wake, an artifact in the data, or some other cosmic quirk.

When the Nancy Grace Roman Telescope launches in 2027, it could help astronomers spot even more runaway supermassive black holes in other galaxies.

“The morphology of the features in the Hubble Space Telescope images is so striking that it should not be too difficult to find more examples, if they exist,” write van Dokkum and his colleagues.