Two supermassive black holes found in collisionally created ‘fossil galaxies’ are so massive that they refuse to collide and merge. The discovery could explain why supermassive black hole mergers, while theoretically predicted, have never been observed over time.
The supermassive black hole system is located in the elliptical galaxy B2 0402+379. Together, the two black holes have a combined mass of 28 billion times larger than that of the Sun, making it the most massive black hole binary ever recorded. Not only that, but the binary components of this system are closest together in a supermassive black hole pair, just 24 light-years apart.
This is the only supermassive black hole binary ever resolved in enough detail to see both objects separately. Strangely, the proximity of the two bodies seems to indicate that they should collide and merge, but it appears that they have been locked in the same orbital dance around each other for more than 3 billion years.
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The team that found the binary in data collected by the Gemini North telescope in Hawaii believes that the supermassive black holes cannot merge due to their enormous mass.
“Normally, it seems that galaxies with lighter pairs of black holes have enough stars and mass to bring the two together quickly,” Roger Romani, a team member and professor of physics at Stanford University, said in a statement. ‘Because this pair is so heavy, it took a lot of stars and gas to get the job done. But the binary star has been scouring the central galaxy for such matter, bringing it to a standstill.”
A supermassive black hole couple is simply not yet compatible
B2 0402+379 is a ‘fossil cluster’ that represents what happens when the amount of stars and gas from an entire cluster of galaxies merge into a single massive galaxy. The enormous mass of the two supermassive black holes at the heart suggests that a chain of mergers between smaller black holes created them when multiple galaxies in the cluster met and merged.
Scientists believe that the core of most, if not all, galaxies is a supermassive black hole with a mass equivalent to millions or billions of suns. No star can collapse and produce such massive black holes. Therefore, supermassive black holes are believed to form through chains of mergers between increasingly larger black holes.
When galaxies themselves collide and merge, scientists theorize that the supermassive black holes at their hearts move together, forming a binary pair. As they orbit each other, these black holes emit ripples in spacetime called gravitational waves that carry angular momentum away from the binary, causing the black holes to spin closer together.
Eventually, when the black holes are close enough together, their gravity should take over, and the black holes collide and merge just like the black holes that collided to create them. The question is: could some supermassive black holes be so big that a collision would stop them?
To better understand this system of black hole heavyweights, the team turned to archival data collected by Gemini North’s Gemini Multi-Object Spectrograph (GSO). This allows them to determine the speed of the stars near the two supermassive black holes, and in turn the total mass of those black holes.
“The excellent sensitivity of GMOS allowed us to map the increasing velocities of the stars as you look closer to the center of the galaxy,” Romani added. ‘This allowed us to deduce the total mass of the black holes that were there.”
A stalled merger
The masses of the two black holes in the system are so large that the team thinks it would take an exceptionally large population of stars around them to bring the supermassive black holes close together. However, as this has happened, the energy leaked from the binary has ejected matter from their surroundings.
This leaves the center of B2 0402+379 devoid of stars and gas close enough to the binary star to leak energy from it. As a result, the progress of these two supermassive black holes towards each other has stalled as they approach the final stage before a merger.
The team’s results provide important context regarding the formation of supermassive black hole binaries following galactic mergers, but also support the idea that the mass of such binaries is integral to keeping black holes from following suit.
The team is currently unsure whether these two supermassive black holes in the most massive binary ever detected will overcome this hiatus to eventually merge, or whether they will be permanently locked in fusion limbo.
“We look forward to further investigations of the core of B2 0402+379, where we will see how much gas is present,” said Tirth Surti, lead author of the study and a Stanford graduate student. “This should give us more insight into whether the supermassive black holes may eventually merge or whether they will remain grounded as a binary.”
One way this supermassive standoff could end is if another galaxy merges with B2 0402+379, bringing many more stars, gas, and another supermassive black hole into the mix and upsetting this delicate balance. However, the fact that B2 0402+379 is a fossil galaxy that has been undisturbed for billions of years makes this scenario likely.
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— Colliding black holes can hide in the light of super-bright quasars
— Two merging supermassive black holes observed at ‘cosmic noon’ in the early universe
One thing this research makes certain is how useful archival data from telescopes like Gemini North, which along with the Gemini South telescope sits on a mountain in the Chilean Andes to form the International Gemini Observatory, are for astronomers.
“The data archive serving the International Gemini Observatory contains a goldmine of untapped scientific discoveries,” said Martin Still, Nation Science Foundation program director for the International Gemini Observatory. “Mass measurements for this extremely supermassive binary black hole are an awe-inspiring example of the potential impact of new research exploring that rich archive.”
The team’s research has been published in the Astrophysical Journal.