A streak of light stretched out from a distant galaxy is the first definite sign of a gargantuan black hole in motion, a new study reports. Putatively, a black hole, fleeing its host galaxy, leaving a trail of rising stars and gas in its motion seems to have been shocked. If confirmed, intergalactic escape could help astronomers learn more about what happens in black holes when galaxies collide.
“It’s a very cool, serendipitous discovery,” says Charlotte Angus, an astrologer at the University of Copenhagen, who was not involved in the new work. “The possibility that this was ejected from a supermassive black hole that was ejected from its own galaxy is very exciting. These were predicted by theory, but there was still little evidence for them.
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While astronomer Pieter van Dokkum and colleagues were watching colliding galaxies with the Hubble Space Telescope, they spotted something peculiar: a long, straight line that seemed to expand from the distant galaxy, growing narrower and brighter as it went.SN: 5/18/22).
“Whatever it is, we haven’t seen it before,” says van Dokkum, of Yale University. “Most astronomical objects are shaped like spirals or blobs.” There are not many lines in the sky. When astronomers see lines, they are usually caused by some movement, such as the view of a satellite telescope passing by (SN: 3/3/23).
To find out, van Dokkum and colleagues followed up the observations with the Keck Observatory in Hawaii. Those observations showed that the streak is associated with a galaxy whose light has been around 8 billion years — more than half the age of the universe — to reach Earth, the team reports in a paper submitted Feb. 9 to arXiv.org. The distance measured is the length of a team line: about 200,000 light years.
That certainly excludes satellites.
“We considered many explanations, and the best fit that we have evidence for is a large object, like a black hole, moving away from the galaxy very quickly,” says van Dokkum.
Their black holes are invisible. But “the black hole leaves the galaxy, it doesn’t leave itself,” says van Dokkum. Some of the stars and gases that are bound by gravity to the black hole leave with it. That gas will emit strong radiation that telescopes can detect. A black hole’s journey through the gas and dust in the outer regions of the galaxy can compress some of the gas into new stars as well, which would also be visible (SN: 7/12/18).
Another possibility is that the line is a beam of radiation sent through the galaxy’s central supermassive black hole. But that mission may lead to a narrow beam when the galaxy is closest and escapes further away. This series 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 nearby black holes. Almost every galaxy has a supermassive black hole at its center. When galaxies merge, their central black holes also eventually merge (SN: 3/5/21). If the conditions are right, that merger can give the black hole a “kick” sent out to fly away at high speed (SN: 4/25/22).
Alternatively, a black hole could spit out one of the three galaxies. As the third galaxy undergoes a permanent merger, three supermassive black holes will be thrown into position. One of the black holes can be thrown out of the galactic cone, while the other two slowly take off in the other direction.
Van Dokkum thinks this happened by accident. The signs are shorter, the series opposite the light and the straight line darker.
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. A more theoretical calculation of what a supermassive runaway black hole looks like would help too.
The discovery prompts Angus to search through the database for more potential black hole streaks. “I wonder if there are more of those records out there, sitting on someone’s data that just got lost,” he said.
Van Dokkum does too. “Now that we know what to look for, these very thin streaks, it makes sense to go back to the Hubble data. We have 25 years of Hubble images that have not been explored for this purpose,” he said. “If more people are found, I think they can do it.”
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