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Two supermassive black holes found in “fossil galaxies” created by collisions are so massive that they refuse to collide and merge. The discovery could explain why, although supermassive black hole mergers are theoretically predicted, they have never been observed in progress.
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 binary black hole ever seen. Not only that, but the binary components of this system are the closest in a pair of supermassive black holes, separated by just 24 light years.
This is the only supermassive black hole binary that has been resolved in enough detail to see both objects separately. Interestingly, while the proximity of the two bodies suggests that they should collide and merge, they appear to have been locked in the same orbital dance with each other for more than 3 billion years.
Related: The pair of supermassive black holes closest to Earth lies in the remains of a galactic collision
The team that found the binary in data collected by the Gemini North telescope in Hawaii believes its tremendous mass prevents supermassive black holes from merging.
“Normally, it appears that galaxies with lighter pairs of black holes have enough stars and mass to bring them together quickly,” team member Roger Romani, a professor of physics at Stanford University, said in a statement. statement. “Because this pair is so heavy, it took a lot of stars and gas to do the job. But the binary has swept the central galaxy of such matter, leaving it stagnant.”
The supermassive black hole pair just isn’t compatible…yet
B2 0402+379 is a “fossil cluster” that represents what happens when the stars and gas of an entire galaxy cluster merge into a single massive galaxy. The tremendous mass of the two supermassive black holes at its 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 at the heart 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 to generate such massive black holes, so supermassive black holes are thought to be born through chains of mergers between increasingly larger black holes.
When galaxies 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 space-time called gravitational waves that shift the angular momentum away from the binary, causing the black holes to orbit closer to each other.
Over time, when the black holes are close enough to each other, their gravitational pull should take over, and the black holes will collide and merge just like the black holes that collided to create them did. The question is: could some supermassive black holes be so massive as to prevent a collision?
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 in the vicinity of 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 we look closer to the center of the galaxy,” Romani added. “With this we were able to infer the total mass of the black holes that reside there“.
A stalled merger
The mass of the system’s two black holes is so large that the team believes it would take an exceptionally large population of stars around them to bring the supermassive black holes closer together. However, while this is happening, the energy leached from the binary has been spewing matter away from its vicinity.
This has left the center of B2 0402+379 devoid of stars and gas close enough to the binary to leach energy from it. As a result, the progress of these two supermassive black holes towards each other has stalled as they approach the final stages before a merger.
The team’s results provide important context on the formation of supermassive black hole binaries after galactic mergers, but also support the idea that the mass of such binaries is critical in preventing black holes from following suit.
Currently, the team is unsure whether these two supermassive black holes in this heaviest binary ever detected will overcome this pause to eventually merge or whether they will be stuck in merger limbo permanently.
“We hope to do follow-up investigations of the core of B2 0402+379, where we will see how much gas is present,” said lead author of the research and Stanford student Tirth Surti. “This should give us more information about whether supermassive black holes may eventually merge or whether they will remain stranded as binaries.”
One way to stop this supermassive showdown is if another galaxy merges with B2 0402+379, which would throw many more stars, gas, and another supermassive black hole into the mix and upset this delicate balance. However, the fact that B2 0402+379 is a fossil galaxy that has not been disturbed for billions of years makes this scenario likely.
One thing this research does ensure is how useful archival data from telescopes like Gemini North, which is combined with the Gemini South telescope located on a mountain in the Chilean Andes, is to astronomers to form the International Gemini Observatory.
“The data archive serving the International Gemini Observatory contains an untapped gold mine of scientific discoveries,” said Martin Still, Nation Science Foundation program director for the International Gemini Observatory. “The mass measurements of this extreme supermassive binary black hole are a stunning example of the potential impact of new research that explores that rich archive.”
The team’s research is published in the Astrophysical Magazine.