Black Hole Hurtling Through Space Leaves a Trail of Stars in Its Wake

Researchers theorize a stream of stars 200,000 light-years long came from a black hole ejected from its galaxy

A runaway black hole with a stream of stars behind it
An artist's rendition of the runaway black hole with the stream of stars trailing behind it. Its former host galaxy is in the upper right of the image. NASA, ESA, Leah Hustak (STScI)

Supermassive black holes are thought to lie at the center of nearly all large galaxies, including our own. But astronomers say they’ve likely identified one that’s doing something quite different—instead of sitting at the heart of a galaxy, it seems to be streaking through the universe at about 3.6 million miles per hour. As it hurtles through space, it’s leaving a trail of stars that stretches 200,000 light-years long.

This rare phenomenon was revealed by chance as astronomers examined an image from the Hubble Space Telescope. The researchers were looking for globular star clusters when they noticed a mysterious smudge. At first, they thought the streak might have been caused by cosmic rays hitting the camera. But follow up observations with the W.M. Keck Observatory in Hawaii confirmed it was a long line of stars, the team reported last week in The Astrophysical Journal Letters.

“We think we’re seeing a wake behind the black hole where the gas cools and is able to form stars,” Pieter van Dokkum, co-author of the new study and an astrophysicist at Yale University, says in a statement from NASA. “What we’re seeing is the aftermath. Like the wake behind a ship, we’re seeing the wake behind the black hole.”

Often the product of giant stars’ deaths, black holes are enormous quantities of mass stuffed into a relatively tiny area. As a result, these ultra-dense points in space have strong gravitational pulls and suck in anything close by, including light. Supermassive black holes are the largest kind and can be as much as a billion times the mass of our sun. The proposed runaway could weigh as much as 20 million suns.

To create such a long trail of stars, any plausible explanation would involve a rapidly moving supermassive black hole, writes Scientific American’s Kenna Hughes-Castleberry. Further, the galaxy near the stream of stars appears not to have a black hole in it, providing more evidence, though circumstantial, to support the theory, per the statement.

An image of the stream of stars
An image containing the stream of stars captured by the Hubble Space Telescope. The theorized black hole would lie at the bottom left of the zoomed-in image, with the stream of stars trailing from the host galaxy in the upper right. NASA, ESA, Pieter van Dokkum (Yale); Image Processing: Joseph DePasquale (STScI)

The researchers estimate that the runaway black hole broke loose from its galaxy roughly 39 million years ago. They theorize the event started when two galaxies merged, linking their respective supermassive black holes in an orbit around each other. Then, a third galaxy and black hole might have approached and destabilized the system, causing one of the black holes to shoot out of its host galaxy at a high speed, per NASA’s statement.

In its wake, the black hole left behind a trail of young, hot blue stars birthed from gas.

“Gas in front of [the black hole] gets shocked because of this supersonic, very high-velocity impact of the black hole moving through the gas. How it works exactly is not really known,” van Dokkum says in the statement.

“Much more data is needed to pinpoint this hypothesis,” Maria Luísa Buzzo, a co-author of the study and an astrophysicist at Swinburne University of Technology in Australia, tells Scientific American. “We need additional data to confirm this scenario and to decrease the uncertainties on the nature of the [stellar stream] feature.”

In the future, the researchers hope observations from the James Webb Space Telescope and the Chandra X-ray Observatory can help test the theory that a black hole created the stream of stars.

“You’d want to see the direct gravitational effects of a compact, massive object,” Erin Bonning, an astronomer at Emory University who did not contribute to the research, tells Scientific American.

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