Many galaxies have a 'supermassive black hole' at their center. These range from hundreds of thousands to billions times the mass of our Sun.
I was shocked to read that astronomers have found evidence for a supermassive black hole shooting out of its host galaxy.
They've seen a long thin feature - apparently a 'wake' of shocked gas and young stars - stretching 200,000 light years from the galaxy's center and ending in a bright object that's putting out 100 million times more power than our Sun.
This is consistent with a supermassive black hole that was thrown out the galactic center at a speed of 1600 kilometers/second, which has been traveling for about 40 million years. This speed is faster than the galactic escape velocity!
But what could be muscular enough to throw a supermassive black hole around?
Only two possibilities are known.
One is another supermassive black hole. When two galaxies collide, their central black holes meet - and may start orbiting each other. If a third galaxy with its own supermassive black hole crashes in, one of the three black holes can get flung out.
That seems quite reasonable to me: galactic collisions are fairly common.
The other possibility is weirder.
When two black holes collide, they can emit gravitational radiation that's beamed mainly in one direction... and this can give them a 'kick' in the opposite direction. I find this surprising in the first place. And it's more surprising that this effect can be big enough to kick a black hole out of a galaxy! But that's what some calculations say.
The paper is here:
https://arxiv.org/abs/2302.04888
and for more about the kick caused by gravitational waves, try this:
"Now, my own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose."
-J. B. S. Haldane in an essay titled “Possible Worlds” published within a 1927 collection.
