Sometimes theoretical science is stranger than science fiction
The most massive black hole ever measured may be an intergalactic hitchhiker that escaped from one galaxy before getting captured by another. If this scenario, laid out in a paper posted February 18 at arXiv.org, is proven correct, it would be the first time astronomers have definitively spotted a black hole that was expelled from its original galactic home.
A supermassive black hole lives in the center of just about every galaxy, including our own Milky Way. Most of the time, these black holes aren’t doing much, but when two galaxies collide — a rather common event — their black holes find each other and merge. Simulations involving relativity predict that under certain circumstances these merging black holes can be flung around and even kicked out of the merging galaxies. To understand how this can happen, we have to consider one of the fundamental properties of a black hole — its spin. Most (perhaps all) black holes are spinning, and this spin has a direction — you can think of black holes as kind of like spinning tops. If you bring two spinning black holes together, and their spins are going in the same direction, the pair coalesces into a single black hole inside of the merged galaxy as this simulation demonstrates:
The disks surrounding each of the black holes in the simulation are probably accretion disks — gaseous material rapidly spiraling down onto the black holes — that, in any case, show the direction of the black hole spin.
But what happens if you bring two spinning black holes together, and their spins are not going in the same direction? Turns out, this will cause the merging pair to flail around, sometimes with enough energy to kick the merged black hole out of the merged galaxy. The kicked black hole could carry gaseous material with it, if the material is gravitationally bound to the black hole, but the black hole would essentially wander the universe homeless.
The study above (authored by two of my long-time collaborators) proposes that the overly massive black hole residing in the relatively modest galaxy NGC 1277 was flung out of the neighboring galaxy, NGC 1275, long ago as it underwent a merger. Instead of wandering the universe homeless, however, the merged black hole pair found a new home in NGC 1277. What makes this scenario appealing is the close correlation between the masses of black holes and the masses of their host galaxies. Black holes are typically about 0.1% of the mass of their hosts, but NGC 1277 is a significant outlier from this relationship: this ho-hum galaxy hosts the most massive black hole ever observed, which weighs in at a stunning 14% of the galaxy’s mass. Adding to the appeal is the fact that NGC 1277 has a close galactic neighbor with a much greater mass, NGC 1275, a more likely original home for the excessively massive black hole.
Now that astrophysicists have a plausible theoretical explanation for NGC 1277’s outsized black hole, the search will be on for observational data supporting this idea.