A runaway black hole has been spotted escaping from a distant galaxy

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A new study reports that a streak of light stretching from a distant galaxy may be the first sure sign of a supermassive black hole. As the putative black hole leaves its host galaxy, it leaves behind a trail of newborn stars and shocked gas. If confirmed, the intergalactic escape could help astronomers learn more about what happens to black holes when galaxies collide.

“This is a very cool, serendipitous discovery,” says astronomer Charlotte Angus of the University of Copenhagen, who was not involved in the new work. “The possibility that it could be related to a supermassive black hole that was ejected from its galaxy is very exciting. These events have been predicted by theory, but until now there has been little evidence for them.”

While searching for colliding dwarf galaxies with the Hubble Space Telescope, astronomer Peter van Dokkum and his colleagues noticed something unusual: a long, straight line that appeared to extend from a distant galaxy, getting narrower and brighter as it went.

“Whatever it is, we haven’t seen it before,” says Yale University’s van Dokkum. “Most astronomical objects have the shape of a spiral or a drop. There aren’t many objects that are just a line in the sky.” When astronomers do see lines, they are usually caused by something moving, such as a satellite crossing the telescope’s field of view.

To understand what this is, van Dokkum and his colleagues conducted further observations at the Keck Observatory in Hawaii. Those observations showed that the streak was associated with a galaxy whose light took about 8 billion years — more than half the age of the universe — to reach Earth, the team reported in a paper submitted Feb. 9 to arXiv.org. Measuring the distance allowed the team to calculate the length of the line: approximately 200,000 light years.

That definitely ruled out a satellite.

“We considered many explanations, and the best fit is that what we observe is a massive object, like a black hole, that is moving very quickly away from the galaxy,” says van Dokkum.

In the Hubble image, the escaping black hole was a straight line (shown). The parent galaxy is in the upper right of the band. The galaxy is so far away that the line stretches for 200,000 light years.P. VAN DOKKUM AND OTHER / ARXIV.ORG 2023

Black holes themselves are invisible. But “if a black hole leaves a galaxy, it doesn’t leave on its own,” says van Dokkum. Some of the stars and gas that were gravitationally bound to the black hole leave it together. This gas will emit strong radiation that can be detected by telescopes. The path of the black hole through the gas and dust in the outer regions of the galaxy may also compress some of this gas into new stars that will also be visible.

Another possibility is that the line is a jet of radiation emitted by the galaxy’s central supermassive black hole. But such a scenario would likely result in a narrow beam when it is close to the galaxy and a widening one as it moves away. This strip does the opposite.

If it is a black hole, it could have been ejected from the center of the galaxy by interacting with one or two other black holes nearby. At the center of almost every galaxy is a supermassive black hole. When galaxies merge, their central black holes also merge over time. If conditions are right, this merger can give the resulting black hole a “push,” sending it flying at high speeds.

In addition, the black hole could have been ejected as a result of the breakup of three galaxies. As a third galaxy joins the ongoing merger, the three supermassive black holes jostle for position. One black hole may be ejected from the galactic smash, while the other two take off more slowly in the other direction.

That’s what van Dokkum believes happened in this case. There are indications of a shorter, dimmer band running in the opposite direction from the bright straight line.

More observations of this system, perhaps with the James Webb Space Telescope, are needed to confirm that it is indeed an ejected supermassive black hole, Angus says. Additional theoretical calculations of what a runaway supermassive black hole should look like would also help.

This finding motivates Angus to search the archived data for more potential black hole streaks. “I wonder if there are other features in someone’s data that might have been missed,” she says.

Van Dokkum too. “Now that we know what to look for, these very thin bands, it makes sense to go back to the Hubble data. We have 25 years of Hubble images that have not been searched for this purpose,” he says. “If we can find more, I think we can do it.”

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