The newly-discovered supermassive black hole resides in a compact star-forming galaxy around 7.7 billion years away and has a mass of 20 million solar masses. The object has left behind a never-before-seen 200,000-light-year-long ‘contrail’ of newborn stars, twice the diameter of our Milky Way Galaxy.
An artist’s impression of a runaway supermassive black hole that was ejected from its host galaxy as a result of a tussle between it and two other black holes. Image credit: NASA / ESA / Leah Hustak, STScI.
There are several ways for a supermassive black hole to escape from the center of a galaxy.
The first step is always a galaxy merger, which leads to the formation of a binary supermassive black hole at the center of the merger remnant.
The binary can be long-lived and 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.
Even in the absence of a third supermassive black hole, the eventual merger of the binary can impart a kick to the newly formed black hole through gravitational radiation recoil.
The velocity of the ejected supermassive black hole depends on the mechanism and the specific dynamics.
Generally the kicks are expected to be higher for slingshot scenarios than for recoils, although in exceptional cases recoils may reach approximately 5,000 km per second.
In both scenarios the velocity of the supermassive black hole may exceed the escape velocity of the host galaxy.
Identifying such runaway supermassive black holes is of obvious interest but difficult.
“We think we’re seeing a wake behind the black hole where the gas cools and is able to form stars,” said Dr. Pieter van Dokkum, an astronomer at Yale University.
“So, we’re looking at star formation trailing the black hole.”
“What we’re seeing is the aftermath. Like the wake behind a ship we’re seeing the wake behind the black hole.”
“The trail must have lots of new stars, given that it is almost half as bright as the host galaxy it is linked to.”
“The black hole lies at one end of the column, which stretches back to its parent galaxy.”
“There is a remarkably bright knot of ionized oxygen at the outermost tip of the column.”
“We believe gas is probably being shocked and heated from the motion of the black hole hitting the gas, or it could be radiation from an accretion disk around the black hole.”
“Gas in front of it 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,” he added.
“This is pure serendipity that we stumbled across it.”
This archival Hubble photo captures a curious linear feature that is so unusual it was first dismissed as an imaging artifact from the telescope’s cameras. But follow-up spectroscopic observations reveal it is a 200,000-light-year-long chain of young blue stars. A supermassive black hole lies at the tip of the bridge at lower left. The black hole was ejected from the galaxy at upper right. It compressed gas in its wake to leave a long trail of young blue stars. Nothing like this has ever been seen before in the Universe. This unusual event happened when the Universe was approximately half its current age. Image credit: NASA / ESA / Pieter van Dokkum, Yale University / Joseph DePasquale, STScI.
Dr. van Dokkum and his colleagues suspect the first two galaxies merged perhaps 50 million years ago.
That brought together two supermassive black holes at their centers. They whirled around each other as a binary black hole.
Then another galaxy came along with its own supermassive black hole.
This follows the old idiom: ‘two’s company and three’s a crowd.’
The three black holes mixing it up led to a chaotic and unstable configuration.
One of the black holes robbed momentum from the other two black holes and got thrown out of the host galaxy.
The original binary may have remained intact, or the new interloper black hole may have replaced one of the two that were in the original binary, and kicked out the previous companion.
When the single black hole took off in one direction, the binary black holes shot off in the opposite direction.
There is a feature seen on the opposite side of the host galaxy that might be the runaway binary black hole.
Circumstantial evidence for this is that there is no sign of an active black hole remaining at the galaxy’s core.
“The next step is to do follow-up observations with the NASA/ESA/CSA James Webb Space Telescope and NASA’s Chandra X-ray Observatory to confirm the black hole explanation.”
A paper on the findings was published in the Astrophysical Journal Letters.
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Pieter van Dokkum et al. 2023. A Candidate Runaway Supermassive Black Hole Identified by Shocks and Star Formation in its Wake. ApJL 946, L50; doi: 10.3847/2041-8213/acba86
