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'Runaway' black hole the size of 20 million suns found speeding through space with a trail of stars

Updated: Apr 25, 2023

Astronomers have discovered a "runaway" black hole, potentially the first observational evidence that supermassive black holes can be ejected from their host galaxies.


An illustration showing a black hole being ejected from a galaxy's center, as a trail of brightly glowing gas follows behind(Image credit: Keio University)


Astronomers have spotted a runaway supermassive black hole, seemingly ejected from its home galaxy and racing through space with a chain of stars trailing 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.


According to the team's research, which was published on the pre-print server arXiv.org(opens in new tab) and has been accepted for publication in The Astrophysical Journal Letters, the discovery offers the first observational evidence that supermassive black holes can be ejected from their home galaxies to roam interstellar space.

The researchers discovered the runaway black hole as a bright streak of light while they were using the Hubble Space Telescope to observe the dwarf galaxy RCP 28, located about 7.5 billion light-years from Earth.


Most, if not all, large galaxies host supermassive black holes at their centers. Active supermassive black holes often launch jets of material at high speeds, which can be seen as streaks of light that superficially resemble the one the researchers spotted. These are called astrophysical jets.

To determine this isn't what they observed, van Dokkum and the team investigated this streak and found it didn't possess any of the telltale signs of an astrophysical jet. While astrophysical jets grow weaker as they move away from their source of emission, the potential supermassive black hole tail actually gets stronger as it progresses away from what seems to be its galactic point of origin, according to the researchers. Also, astrophysical jets launched by black holes fan out from their source, whereas this trail seems to have remained linear.

The team concluded that the explanation that best fits the streak is a supermassive black hole blasting through the gas that surrounds its galaxy while compressing that gas enough to trigger star formation in its wake.

"If confirmed, it would be the first time that we have clear evidence that supermassive black holes can escape from galaxies," van Dokkum said.


"Ejected supermassive black holes had been predicted for 50 years but none have been unambiguously seen," van Dokkum said "Most theorists think that there should be many out there."

Further observations with other telescopes are needed to find direct evidence of a black hole at the mysterious streak's tip, van Dokkum added.


According to the team's research, which was published on the pre-print server arXiv.org(opens in new tab) and has been accepted for publication in The Astrophysical Journal Letters, the discovery offers the first observational evidence that supermassive black holes can be ejected from their home galaxies to roam interstellar space.



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.


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.



Sources:



CAA Team




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