Two supermassive black holes found in collision-created “fossil galaxies” are so massive that they refuse to collide and merge. The discovery could explain why, although supermassive black hole mergers are predicted theoretically, they have never been observed in progress.
The supermassive black hole system is located in elliptical galaxy B2 0402+379. Together, the two black holes have a joint mass that is 28 billion times larger than that of the sun, making this the most massive black hole binary ever seen. Not only that, but the binary components of this system are the closest in a supermassive black hole pair, separated by just 24 light-years.
This is the only supermassive black hole binary that has ever been resolved in enough detail to see both objects separately. Curiously, while the proximity of the two bodies suggests they should collide and merge, they appear to have been locked in the same orbital dance around each other for over 3 billion years.
The team that found the binary thinks the supermassive black holes are being prevented from merging by their tremendous mass.
“Normally, it seems that galaxies with lighter black hole pairs have enough stars and mass to drive the two together quickly,” said Roger Romani, a member of the team and physics professor at Stanford University. “Since this pair is so heavy, it required lots of stars and gas to get the job done. But the binary has scoured the central galaxy of such matter, leaving it stalled.”
Supermassive black hole couple just isn’t compatible… yet
B2 0402+379 is a “fossil cluster” that represents what happens when an entire galaxy cluster’s worth of stars and gas merge into one 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 as multiple galaxies in the cluster met and merged together.
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 single star can collapse to birth such massive black holes, so it is believed that supermassive black holes are born via chains of mergers between successively larger and larger black holes.